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• Hardware specs for web cache

  - by Raj

  I am looking for recommendations for hardware specs for a server that needs to be a web cache for a user population of about 2,000 concurrent connections. The clients are viewing segmented HTTP video in bitrates ranging from 150kbps to 2mbps. Most video is "live" meaning segments of 2-10 secs each, of which 100 or so are maintained at a time. There are also some pre-recorded fixed length videos. How would I go about doing the provisioning calculation for such a server: What kind of HDD (SSD?), how many NICs how much RAM etc? I am thinking of using Varnish on Linux, all the RAM I can get my hands on, 2 CPUs with 6-8 cores each.

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• How to monitor nginx proxy cache?

  - by Isaac

  I would like to see which objects get cached by my nginx reverse proxy (with an apache as a backend). So far I could not find a way, only the info that its not implemented yet. The reason is that I would like to tweak my configuration for best performance without putting too much stress on the server, as the backend is a production system. I know benchmarking would be better, but its not an option right now. So I though an alternative measure would be to monitor the cache. Is that possible, and if yes, how? (despite patching nginx with the patch mentioned in the link above)

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• Cache that always returns immediate response?

  - by Col Wilson

  I have a web service that takes a while to build a response despite being tuned as best I can. What I'd like is some sort of cache sitting in front of the service which would always return the last known value from the service, but at the same time pass the request back to the service to build an up to date response for the next request. I'm aware of the limitations that this puts on the freshness of the data, but you can assume that I'm happy to live with that. The technologies I'm using at present are python uwsgi via nginx, but that need not be a limit to any solution you might suggest. Col

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• svn:externals cache and stale URLs

  - by dcaunt

  I have a subversion externals entry in a library folder which looks like this: Z https://svn/Z/trunk/library/Z Fetching external item into '/home/releases/50/library/Z' svn: OPTIONS of 'http://svn/repo/trunk/library/Z': could not connect to server (http://svn) The externals URL was the same, but over the HTTP protocol. Having changed the externals to point to the HTTPS, I can't figure out why subversion is still trying to use the old URL. Does subversion cache the externals path, and if so how can I clear this? If not, what else could be causing this? I can check out from the correct (HTTPS) URL fine from the server. NOTE: svn is an entry in the server's local hosts file, pointing to our subversion server's IP.

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• How does key-based caching work?

  - by Dominic Santos

  I recently read an article on the 37Signals blog and I'm left wondering how it is that they get the cache key. It's all well and good having a cache key that includes the object's timestamp (this means that when you update the object the cache will be invalidated); but how do you then use the cache key in a template without causing a DB hit for the very object that you are trying to fetch from the cache. Specifically, how does this affect One to Many relations where you are rendering a Post's Comments for example. Example in Django: {% for comment in post.comments.all %} {% cache comment.pk comment.modified %} <p>{{ post.body }}</p> {% endcache %} {% endfor %} Is caching in Rails different to just requests to memcached for example (I know that they convert your cache key to something different). Do they also cache the cache key?

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• Login screen appears even if logged in

  - by Prasenjit

  HEHE it's a stupid problem, for those who care place the following on the load event of the master page or default             Response.AppendHeader("Cache-Control", "no-cache"); //HTTP 1.1             Response.AppendHeader("Cache-Control", "private"); // HTTP 1.1             Response.AppendHeader("Cache-Control", "no-store"); // HTTP 1.1             Response.AppendHeader("Cache-Control", "must-revalidate"); // HTTP 1.1             Response.AppendHeader("Cache-Control", "max-stale=0"); // HTTP 1.1              Response.AppendHeader("Cache-Control", "post-check=0"); // HTTP 1.1              Response.AppendHeader("Cache-Control", "pre-check=0"); // HTTP 1.1              Response.AppendHeader("Pragma", "no-cache"); // HTTP 1.1              Response.AppendHeader("Keep-Alive", "timeout=3, max=993"); // HTTP 1.1              Response.AppendHeader("Expires", "Mon, 26 Jul 1997 05:00:00 GMT"); // HTTP 1.1   It finally worked for me:)

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• how to create a system-wide independent universal counter object primarily for Database keys?

  - by andora

  I would like to create/use a system-wide independent universal 'counter object' that can be called via COM in a thread-safe manner. The counter object will be passed an ID to identify which counter to return, handle the counting, 'persist' the count (occasionally), have reasonable performance (as fast as possible) perhaps capable of 1000 counts per second or better (1mS) and be accessible cross-process/out-of-process. The current count status must be persisted between object restarts/shutdowns. The counter object is liklely to be a 'singleton' type object implemented in some form of free-threaded dictionary, containing maybe 10 counters (perhaps 50 max). The count needs to be monotonic and consistent, (ie: guaranteed unique sequential values). Each counter should have a few methods, like reset, inc, dec, set, clear, remove. As a luxury, I would like to have a variable-increment (ie: 'step by' value). To support thread-safefty, perhaps some sorm of critical-section or mutex call. It just needs to return a long/4byte signed integer. I really want something that can be called from anywhere, including VBScript, so I figure COM is my preferred solution. The primary use of this is for database keys. I am unable to use autoinc or guid type keys and have ruled out database-generated counting systems at this point. I've spent days researching this and I have really struggled to find a solution. The best I can find is a free-threaded dictionary object that can be instantiated using COM+ from Motobit - it seems to offer all the 'basics' and I guess I could create some form of wrapper for this. So, here are my questions: Does such a 'general purpose counter-object already exist? Can you direct me to it? (MS did do an IIS/ASP object called 'MSWC.Counter' but this isn't 'cross-process'/ out-of-process component and isn't thread-safe. (but if it was, it would do!) What is the best way of creating such a Component? (I'd prefer VB6 right-now, [don't ask!] but can do in VB.NET2005 if I had to). I don't have the skills/knowledge/tools to use anything else. I am desparate for a workable solution. I need specific guidance! If anybody can code something up for me I am prepared to pay for it.

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• Java: Combination of recursive loops which has different FOR loop inside; Output: FOR loops indexes

  - by vvinjj

  currently recursion is fresh & difficult topic for me, however I need to use it in one of my algorithms. Here is the challenge: I need a method where I specify number of recursions (number of nested FOR loops) and number of iterations for each FOR loop. The result should show me, something simmilar to counter, however each column of counter is limited to specific number. ArrayList<Integer> specs= new ArrayList<Integer>(); specs.add(5); //for(int i=0 to 5; i++) specs.add(7); specs.add(9); specs.add(2); specs.add(8); specs.add(9); public void recursion(ArrayList<Integer> specs){ //number of nested loops will be equal to: specs.size(); //each item in specs, specifies the For loop max count e.g: //First outside loop will be: for(int i=0; i< specs.get(0); i++) //Second loop inside will be: for(int i=0; i< specs.get(1); i++) //... } The the results will be similar to outputs of this manual, nested loop: int[] i; i = new int[7]; for( i[6]=0; i[6]<5; i[6]++){ for( i[5]=0; i[5]<7; i[5]++){ for(i[4] =0; i[4]<9; i[4]++){ for(i[3] =0; i[3]<2; i[3]++){ for(i[2] =0; i[2]<8; i[2]++){ for(i[1] =0; i[1]<9; i[1]++){ //... System.out.println(i[1]+" "+i[2]+" "+i[3]+" "+i[4]+" "+i[5]+" "+i[6]); } } } } } } I already, killed 3 days on this, and still no results, was searching it in internet, however the examples are too different. Therefore, posting the programming question in internet first time in my life. Thank you in advance, you are free to change the code efficiency, I just need the same results.

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• Building "isolated" and "automatically updated" caches (java.util.List) in Java.

  - by Aidos

  Hi Guys, I am trying to write a framework which contains a lot of short-lived caches created from a long-living cache. These short-lived caches need to be able to return their entier contents, which is a clone from the original long-living cache. Effectively what I am trying to build is a level of transaction isolation for the short-lived caches. The user should be able to modify the contents of the short-lived cache, but changes to the long-living cache should not be propogated through (there is also a case where the changes should be pushed through, depending on the Cache type). I will do my best to try and explain: master-cache contains: [A,B,C,D,E,F] temporary-cache created with state [A,B,C,D,E,F] 1) temporary-cache adds item G: [A,B,C,D,E,F] 2) temporary-cache removes item B: [A,C,D,E,F] master-cache contains: [A,B,C,D,E,F] 3) master-cache adds items [X,Y,Z]: [A,B,C,D,E,F,X,Y,Z] temporary-cache contains: [A,C,D,E,F] Things get even harder when the values in the items can change and shouldn't always be updated (so I can't even share the underlying object instances, I need to use clones). I have implemented the simple approach of just creating a new instance of the List using the standard Collection constructor on ArrayList, however when you get out to about 200,000 items the system just runs out of memory. I know the value of 200,000 is excessive to iterate, but I am trying to stress my code a bit. I had thought that it might be able to somehow "proxy" the list, so the temporary-cache uses the master-cache, and stores all of it's changes (effectively a Memento for the change), however that quickly becomes a nightmare when you want to iterate the temporary-cache, or retrieve an item at a specific index. Also given that I want some modifications to the contents of the list to come through (depending on the type of the temporary-cache, whether it is "auto-update" or not) and I get completly out of my depth. Any pointers to techniques or data-structures or just general concepts to try and research will be greatly appreciated. Cheers, Aidos

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• Need help profiling .NET caching extension method.

  - by rockinthesixstring

  I've got the following extension Public Module CacheExtensions Sub New() End Sub Private sync As New Object() Public Const DefaultCacheExpiration As Integer = 1200 ''# 20 minutes <Extension()> Public Function GetOrStore(Of T)(ByVal cache As Cache, ByVal key As String, ByVal generator As Func(Of T)) As T Return cache.GetOrStore(key, If(generator IsNot Nothing, generator(), Nothing), DefaultCacheExpiration) End Function <Extension()> Public Function GetOrStore(Of T)(ByVal cache As Cache, ByVal key As String, ByVal generator As Func(Of T), ByVal expireInSeconds As Double) As T Return cache.GetOrStore(key, If(generator IsNot Nothing, generator(), Nothing), expireInSeconds) End Function <Extension()> Public Function GetOrStore(Of T)(ByVal cache As Cache, ByVal key As String, ByVal obj As T) As T Return cache.GetOrStore(key, obj, DefaultCacheExpiration) End Function <Extension()> Public Function GetOrStore(Of T)(ByVal cache As Cache, ByVal key As String, ByVal obj As T, ByVal expireInSeconds As Double) As T Dim result = cache(key) If result Is Nothing Then SyncLock sync If result Is Nothing Then result = If(obj IsNot Nothing, obj, Nothing) cache.Insert(key, result, Nothing, DateTime.Now.AddSeconds(expireInSeconds), cache.NoSlidingExpiration) End If End SyncLock End If Return DirectCast(result, T) End Function End Module From here, I'm using the extension is a TagService to get a list of tags Public Function GetTagNames() As List(Of String) Implements Domain.ITagService.GetTags ''# We're not using a dynamic Cache key because the list of TagNames ''# will persist across all users in all regions. Return HttpRuntime.Cache.GetOrStore(Of List(Of String))("TagNamesOnly", Function() _TagRepository.Read().Select(Function(t) t.Name).OrderBy(Function(t) t).ToList()) End Function All of this is pretty much straight forward except when I put a breakpoint on _TagRepository.Read(). The problem is that it is getting called on every request, when I thought that it is only to be called when Result Is Nothing Am I missing something here?

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• Why does using ASP.NET OutputCache keep returning a 200 OK, not a 304 Not Modified?

  - by Pure.Krome

  Hi folks, i have a simple aspx page. Here's the top of it:- <%@ Page Language="C#" AutoEventWireup="true" CodeFile="Foo.aspx.cs" Inherits="Foo" %> <%@ OutputCache Duration="3600" VaryByParam="none" Location="Any" %> Now, every time I hit the page in FireFox (either hit F5 or hit enter in the url bar) I keep getting a 200 OK response. Here's a sample reply from FireBug :- Request Headers:- GET /sitemap.xml HTTP/1.1 Host: localhost.foo.com.au User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.2) Gecko/20100115 Firefox/3.6 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-au,en-gb;q=0.7,en;q=0.3 Accept-Encoding: gzip,deflate Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive: 115 Connection: keep-alive Cookie: <snipped> If-Modified-Since: Tue, 01 Jun 2010 07:35:17 GMT If-None-Match: "" Cache-Control: max-age=0 Response Headers:- HTTP/1.1 200 OK Cache-Control: public Content-Type: text/xml; charset=utf-8 Expires: Tue, 01 Jun 2010 08:35:17 GMT Last-Modified: Tue, 01 Jun 2010 07:35:17 GMT Etag: "" Server: Microsoft-IIS/7.5 X-Powered-By: UrlRewriter.NET 2.0.0 X-AspNet-Version: 4.0.30319 Date: Tue, 01 Jun 2010 07:35:20 GMT Content-Length: 775 Firebug Cache tab:- Last Modified Tue Jun 01 2010 17:35:20 GMT+1000 (AUS Eastern Standard Time) Last Fetched Tue Jun 01 2010 17:35:20 GMT+1000 (AUS Eastern Standard Time) Expires Tue Jun 01 2010 18:35:17 GMT+1000 (AUS Eastern Standard Time) Data Size 775 Fetch Count 105 Device disk Now, if i try it in Fiddler using the Request Builder (and no extra data) I also keep getting the same 200 OK reply. Request Headers:- GET http://localhost.foo.com.au/sitemap.xml HTTP/1.1 User-Agent: Fiddler Host: foo.com.au Response Headers:- HTTP/1.1 200 OK Cache-Control: public Content-Type: text/xml; charset=utf-8 Expires: Tue, 01 Jun 2010 07:58:00 GMT Last-Modified: Tue, 01 Jun 2010 06:58:00 GMT ETag: "" Server: Microsoft-IIS/7.5 X-Powered-By: UrlRewriter.NET 2.0.0 X-AspNet-Version: 4.0.30319 Date: Tue, 01 Jun 2010 06:59:16 GMT Content-Length: 775 It looks like it's asking to cache it but it's not :( Server is a localhost IIS7.5 on Win7. (as listed in the Response data). Can anyone see what I'm doing wrong?

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• Impact of the L3 cache on performance - worth a dual-processor system?

  - by Dan Nissenbaum

  I will be purchasing a new high-end system, and I would like to have a better sense of whether a dual-processor Xeon system (I am looking at the new, high-end Xeon E5-2687W) might, realistically, provide a noticeable performance improvement due to the doubling of the L3 cache (20 MB per CPU). (This is in addition to the occasional added advantage due to the doubling of cores and RAM.) My usage scenario is, roughly, that I have many background applications running at any time - 3 or 4 data compression/backup applications, a low-impact web server, one or two virtual machines at any given time (usually fairly idle), and perhaps 20 utility programs that utilize a noticeable (but small) portion of the CPU cores. In total, when I am not actively using the computer, about 25% of the total CPU power is utilized in my current i7-970 6-core (12 thread) system. When I am doing routine work, the CPU utilization often exceeds 50%, and occasionally hits 75%-80%. The Xeon E5-2687W is not only a second-generation i7 (so should improve performance for that reason), but also has 8 cores (16 threads), rather than 6 cores. For this reason, I expect to run into the 75% CPU range even less frequently. Nonetheless, the ability to double the cores and the RAM is a consideration. However, in the end, I believe this decision comes down to whether the doubling of the L3 cache will provide a noticeable improvement. There are many benchmarks, and a lot of discussion, regarding CPU power. However, I find very little discussion of L3 cache utilization, and how increases in the L3 cache (such as doubling it with dual processors) affect performance. For example: If there are only two processes running, but each benefits from a large L3 cache (such as might be the case for background processes that frequently scan the file system), perhaps the overall system performance might noticeably improve with dual CPU's - even if only a single core is active on each CPU - due to each process having double the effective L3 cache. I am hoping that someone has a sense of the benefits of increasing (or doubling) the L3 cache size. Note: the CPU I am considering (the Xeon E5-2687W) has 20 MB L3 cache, so a system with dual CPU's would have 40 MB L3 cache.

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• Why are some web clients requesting a page named "cache"?

  - by Toto

  We see errors like this in the apache error log: [Thu May 17 14:32:35 2012] [error] [client 192.168.1.1] File does not exist: /home/www-data/mywebsite.com/r/cache, referer: http://www.mywebsite.com/r/1010 It is strange because: There is no reference in the code/url about a folder/file "cache". The folder/file "cache" does not exist The client is randomly trying to access a "cache" folder everywhere on the website. It is always trying to access the folder/file "cache" following this pattern: Pattern: /level1/.../levelwhatever/filename (referer) /level1/.../levelwhatever/cache We run a LAMP (Debian stable: PHP 5.3.3-7+squeeze9. We also use APC 3.1.3p1). We use Google Analytics and AdSense. We do not know how to reproduce the problem. Note: I replaced the user's IP in the code for privacy.

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• Powerpoint: a slide that counts to 2010

  - by flybywire

  I want a slide in powerpoint that shows the number 0 and when I click a button the number increases by one one until 2010. It should take 4 to 6 seconds. How can I do that?

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• asp mvc unit test HttpContext.Current.Cache?

  - by Paul Creasey

  Here is the first part of my controller code: public class ControlMController : Controller { IControlMService _controlMservice; public IList<User> Users { get { if (System.Web.HttpContext.Current.Cache["users"] == null) { System.Web.HttpContext.Current.Cache["users"] = _controlMservice.GetUsers(); } return (IList<User>)System.Web.HttpContext.Current.Cache["users"]; } } public ControlMController(IControlMService controlMservice) { this._controlMservice = controlMservice; var users = Users; ViewData["Users"] = users; ViewData["jqSelectUsers"] = string.Join(";", users.Select(x => x.UserID + ":" + x.Name).ToArray()); } I'm trying to test it, and because i'm caching using the HttpContext, i'm struggling with null reference exceptions. I've tried using MvcContrib.TestHelper; here is my sample test... [TestMethod] public void EventDetails_Returns_view_with_correct_event() { var builder = new TestControllerBuilder(); var controller = builder.CreateController<ControlMController>( new ControlMService( new MockControlMRepository() )); var view = (controller.EventDetails(1) as ViewResult); Assert.AreEqual(1, (view.ViewData.Model as Event).EventId); } (I haven't quite got round to using DI for my tests! I'm still getting the same null reference exception when the code hits the httpcontext: Error 1 TestCase 'SupportTool.Tests.Services.ControlM.ControlMControllerTests.EventDetails_Returns_view_with_correct_event' failed: System.NullReferenceException: Object reference not set to an instance of an object. at SupportTool.web.Controllers.ControlMController.get_Users() Any ideas?

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• How to cache an HTTP POST response?

  - by KARASZI István

  I would like to create a cacheable HTTP response for a POST request. My actual implementation responses the following for the POST request: HTTP/1.1 201 Created Expires: Sat, 03 Oct 2020 15:33:00 GMT Cache-Control: private,max-age=315360000,no-transform Content-Type: application/x-www-form-urlencoded; charset=UTF-8 Content-Length: 9 ETag: 2120507660800737950 Last-Modified: Wed, 06 Oct 2010 15:33:00 GMT ......... But it looks like that the browsers (Safari, Firefox tested) are not cacheing the response. In the HTTP RFC the corresponding part says: Responses to this method are not cacheable, unless the response includes appropriate Cache-Control or Expires header fields. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cacheable resource. So I think it should be cached. I know I could set a session variable and set a cookie and do a 303 redirect, but I want to cache the response of the POST request. Is there any way to do this? P.S.: I've started with a simple 200 OK, so it does not work. Thanks,

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• InstallExecuteSequence cache interferes with custom action operation

  - by Dima G

  I need to upgrade a product that could be installed in per-user context to a new version that is always in per-machine context. The requirements are: Whether the old version was installed in a Per-User (no matter who) or Per-Machine context should be completely seamless to an administrator user that performs the upgrade. The MSI upgrade should succeed without the need to know the password of the user that originally installed the previous version of the product in a Per-User context. The installation should be performed from a single .msi file (no setup.exe is allowed). The installation should be able to run in a silent (non-UI) mode. No reboots are allowed during installation. My strategy is to find in the beginning of the installation whether the product is already installed in per-user context, and if so, to transform the registry keys manually to Per-Machine context (I checked: no additional changes such as file system changes etc. are needed except this transform). I figured out how to move all appropriate keys in the registry from the user settings to the machine settings (pre-loaded appropriate user hive in case it didn't appear in HKEY_USERS) and created custom action that does it - and it does work when I run it as a stand-alone executable before running the MSI. The problem, however, is that when Windows Installer runs InstallExecuteSequence it first creates a 'cached product context' for all products. So when my custom action runs in the course of InstallExecuteSequence, this cache has already been created. Thus FindRelatedProducts action that determines if older product with same upgrade code exists looks on that cache and ignores the changes that my custom action applied. If before running the MSI the previous product was in per-user context, FindRelatedProducts will look at the cache and not apply the upgrade and remove the previous version, because the new product is in per-machine context, even though the previous product version is already configured to per-machine context in the registry by that time by my custom action. What can be done to solve this problem without violating the requirements stated above?

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• newbie hibernate first level cache confusion

  - by Bruce

  Hi all I'm just geting to grips with hibernate. Little bit confused. I just wanted to watch the operation of the first level cache, which I understood to batch up queries until the end of the session. But if I create an object, hibernate saves it immediately, so that when I later update it in the same transaction, it has to do an update too: Session session = factory.getCurrentSession(); session.beginTransaction(); Test1 test1 = new Test1(); test1.setName("Test 1"); test1.setValue(10); // Touch it session.save(test1); System.out.println("At checkpoint 1"); test1.setValue(20); session.getTransaction().commit(); I see the sql for the save, then 'At checkpoint 1', then the sql for the update. Do I have something set up wrong or am I misunderstanding hibernate's first level cache? Is there a good document on the first level cache - I didn't find anything in the hibernate docs, but I could easily have missed it.. Thanks!

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• Best approach to cache Counts from SQL tables ?

  - by pixel3cs

  I would like to develop a Forum from scratch, with special needs and customization. I would like to prepare my forum for intensive usage and wondering how to cache things like User posts count and User replies count. Having only three tables, tblForum, tblForumTopics, tblForumReplies, what is the best approach of cache the User topics and replies counts ? Think at a simple scenario: user press a link and open the Replies.aspx?id=x&page=y page, and start reading replies. On the HTTP Request, the server will run an SQL command wich will fetch all replies for that page, also "inner joining with tblForumReplies to find out the number of User replies for each user that replied." select tblForumReplies.*, tblFR.TotalReplies from tblForumReplies inner join ( select IdRepliedBy, count(*) as TotalReplies from tblForumReplies group by IdRepliedBy ) as tblFR on tblFR.IdRepliedBy = tblForumReplies.IdRepliedBy Unfortunately this approach is very cpu intensive, and I would like to see your ideas of how to cache things like table Counts. If counting replies for each user on insert/delete, and store it in a separate field, how to syncronize with manual data changing. Suppose I will manually delete Replies from SQL.

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• solaris zpool SSD cache device "faulted"

  - by John-ZFS

  I am trying to get over these SATA SSD errors - smartctl command failed to read the SATA SSD - SATA is not supported what could be the reason for errors? does this mean that SSD has reached EOL & needs to be replacement? errors: No known data errors pool: zpool1216 state: DEGRADED status: One or more devices are faulted in response to persistent errors. Sufficient replicas exist for the pool to continue functioning in a degraded state. action: Replace the faulted device, or use 'zpool clear' to mark the device repaired. scan: scrub repaired 0 in 0h24m with 0 errors on Fri May 18 14:31:08 2012 config: NAME STATE READ WRITE CKSUM zpool1216 DEGRADED 0 0 0 raidz1-0 ONLINE 0 0 0 c11t10d0 ONLINE 0 0 0 c11t11d0 ONLINE 0 0 0 c11t12d0 ONLINE 0 0 0 c11t13d0 ONLINE 0 0 0 c11t14d0 ONLINE 0 0 0 c11t15d0 ONLINE 0 0 0 c11t16d0 ONLINE 0 0 0 c11t1d0 ONLINE 0 0 0 c11t2d0 ONLINE 0 0 0 c11t3d0 ONLINE 0 0 0 c11t4d0 ONLINE 0 0 0 c11t5d0 ONLINE 0 0 0 c11t6d0 ONLINE 0 0 0 c11t7d0 ONLINE 0 0 0 c11t8d0 ONLINE 0 0 0 c11t9d0 ONLINE 0 0 0 logs c9d0 FAULTED 0 0 0 too many errors cache c10d0 FAULTED 0 17 0 too many errors

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• Can't access my accelerated hard disk from msdos after installing linux on ssd cache

  - by Chibueze Opata

  I mistakenly installed Linux Mint on my ssd (forgot my PC actually came with one), when it detected a ~31GiB disk that it wanted to install to, I was a bit confused since I had brought out 30Gb in my primary disk for it, but I clicked continue. After installation, I tried to boot back into my Windows and it brought out some Intel Raid Disk Utility stuff saying I should disable acceleration on a disk something couldn't be found, I canceled it but whatever I tried, recovery tools, setups etc, I couldn't just access the drive which was apparently using the SSD as cache. Since then I've been stuck. I tried setting the 'raid' flag to the disk from 'gParted', still I couldn't. I tried the diskraid utility from windows recover disk, it said it couldn't detect any raid, diskpart sees the partition but doesn't see the volume, when I remove the raid flag, it sees the volume as one of raw type, and I can't access anything. I can however mount the drive from terminal in Mint and access my files, but I don't have any backup media at the moment so I can do a factory re-install. Please how do I go about solving the issue, precisely I would like to know how to boot into the drive again. Thanks!

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• .NET Code Evolution

  - by Alois Kraus

  Originally posted on: http://geekswithblogs.net/akraus1/archive/2013/07/24/153504.aspxAt my day job I do look at a lot of code written by other people. Most of the code is quite good and some is even a masterpiece. And there is also code which makes you think WTF… oh it was written by me. Hm not so bad after all. There are many excuses reasons for bad code. Most often it is time pressure followed by not enough ambition (who cares) or insufficient training. Normally I do care about code quality quite a lot which makes me a (perceived) slow worker who does write many tests and refines the code quite a lot because of the design deficiencies. Most of the deficiencies I do find by putting my design under stress while checking for invariants. It does also help a lot to step into the code with a debugger (sometimes also Windbg). I do this much more often when my tests are red. That way I do get a much better understanding what my code really does and not what I think it should be doing. This time I do want to show you how code can evolve over the years with different .NET Framework versions. Once there was  time where .NET 1.1 was new and many C++ programmers did switch over to get rid of not initialized pointers and memory leaks. There were also nice new data structures available such as the Hashtable which is fast lookup table with O(1) time complexity. All was good and much code was written since then. At 2005 a new version of the .NET Framework did arrive which did bring many new things like generics and new data structures. The “old” fashioned way of Hashtable were coming to an end and everyone used the new Dictionary<xx,xx> type instead which was type safe and faster because the object to type conversion (aka boxing) was no longer necessary. I think 95% of all Hashtables and dictionaries use string as key. Often it is convenient to ignore casing to make it easy to look up values which the user did enter. An often followed route is to convert the string to upper case before putting it into the Hashtable. Hashtable Table = new Hashtable(); void Add(string key, string value) { Table.Add(key.ToUpper(), value); } This is valid and working code but it has problems. First we can pass to the Hashtable a custom IEqualityComparer to do the string matching case insensitive. Second we can switch over to the now also old Dictionary type to become a little faster and we can keep the the original keys (not upper cased) in the dictionary. Dictionary<string, string> DictTable = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase); void AddDict(string key, string value) { DictTable.Add(key, value); } Many people do not user the other ctors of Dictionary because they do shy away from the overhead of writing their own comparer. They do not know that .NET has for strings already predefined comparers at hand which you can directly use. Today in the many core area we do use threads all over the place. Sometimes things break in subtle ways but most of the time it is sufficient to place a lock around the offender. Threading has become so mainstream that it may sound weird that in the year 2000 some guy got a huge incentive for the idea to reduce the time to process calibration data from 12 hours to 6 hours by using two threads on a dual core machine. Threading does make it easy to become faster at the expense of correctness. Correct and scalable multithreading can be arbitrarily hard to achieve depending on the problem you are trying to solve. Lets suppose we want to process millions of items with two threads and count the processed items processed by all threads. A typical beginners code might look like this: int Counter; void IJustLearnedToUseThreads() { var t1 = new Thread(ThreadWorkMethod); t1.Start(); var t2 = new Thread(ThreadWorkMethod); t2.Start(); t1.Join(); t2.Join(); if (Counter != 2 * Increments) throw new Exception("Hmm " + Counter + " != " + 2 * Increments); } const int Increments = 10 * 1000 * 1000; void ThreadWorkMethod() { for (int i = 0; i < Increments; i++) { Counter++; } } It does throw an exception with the message e.g. “Hmm 10.222.287 != 20.000.000” and does never finish. The code does fail because the assumption that Counter++ is an atomic operation is wrong. The ++ operator is just a shortcut for Counter = Counter + 1 This does involve reading the counter from a memory location into the CPU, incrementing value on the CPU and writing the new value back to the memory location. When we do look at the generated assembly code we will see only inc dword ptr [ecx+10h] which is only one instruction. Yes it is one instruction but it is not atomic. All modern CPUs have several layers of caches (L1,L2,L3) which try to hide the fact how slow actual main memory accesses are. Since cache is just another word for redundant copy it can happen that one CPU does read a value from main memory into the cache, modifies it and write it back to the main memory. The problem is that at least the L1 cache is not shared between CPUs so it can happen that one CPU does make changes to values which did change in meantime in the main memory. From the exception you can see we did increment the value 20 million times but half of the changes were lost because we did overwrite the already changed value from the other thread. This is a very common case and people do learn to protect their  data with proper locking.   void Intermediate() { var time = Stopwatch.StartNew(); Action acc = ThreadWorkMethod_Intermediate; var ar1 = acc.BeginInvoke(null, null); var ar2 = acc.BeginInvoke(null, null); ar1.AsyncWaitHandle.WaitOne(); ar2.AsyncWaitHandle.WaitOne(); if (Counter != 2 * Increments) throw new Exception(String.Format("Hmm {0:N0} != {1:N0}", Counter, 2 * Increments)); Console.WriteLine("Intermediate did take: {0:F1}s", time.Elapsed.TotalSeconds); } void ThreadWorkMethod_Intermediate() { for (int i = 0; i < Increments; i++) { lock (this) { Counter++; } } } This is better and does use the .NET Threadpool to get rid of manual thread management. It does give the expected result but it can result in deadlocks because you do lock on this. This is in general a bad idea since it can lead to deadlocks when other threads use your class instance as lock object. It is therefore recommended to create a private object as lock object to ensure that nobody else can lock your lock object. When you read more about threading you will read about lock free algorithms. They are nice and can improve performance quite a lot but you need to pay close attention to the CLR memory model. It does make quite weak guarantees in general but it can still work because your CPU architecture does give you more invariants than the CLR memory model. For a simple counter there is an easy lock free alternative present with the Interlocked class in .NET. As a general rule you should not try to write lock free algos since most likely you will fail to get it right on all CPU architectures. void Experienced() { var time = Stopwatch.StartNew(); Task t1 = Task.Factory.StartNew(ThreadWorkMethod_Experienced); Task t2 = Task.Factory.StartNew(ThreadWorkMethod_Experienced); t1.Wait(); t2.Wait(); if (Counter != 2 * Increments) throw new Exception(String.Format("Hmm {0:N0} != {1:N0}", Counter, 2 * Increments)); Console.WriteLine("Experienced did take: {0:F1}s", time.Elapsed.TotalSeconds); } void ThreadWorkMethod_Experienced() { for (int i = 0; i < Increments; i++) { Interlocked.Increment(ref Counter); } } Since time does move forward we do not use threads explicitly anymore but the much nicer Task abstraction which was introduced with .NET 4 at 2010. It is educational to look at the generated assembly code. The Interlocked.Increment method must be called which does wondrous things right? Lets see: lock inc dword ptr [eax] The first thing to note that there is no method call at all. Why? Because the JIT compiler does know very well about CPU intrinsic functions. Atomic operations which do lock the memory bus to prevent other processors to read stale values are such things. Second: This is the same increment call prefixed with a lock instruction. The only reason for the existence of the Interlocked class is that the JIT compiler can compile it to the matching CPU intrinsic functions which can not only increment by one but can also do an add, exchange and a combined compare and exchange operation. But be warned that the correct usage of its methods can be tricky. If you try to be clever and look a the generated IL code and try to reason about its efficiency you will fail. Only the generated machine code counts. Is this the best code we can write? Perhaps. It is nice and clean. But can we make it any faster? Lets see how good we are doing currently. Level Time in s IJustLearnedToUseThreads Flawed Code Intermediate 1,5 (lock) Experienced 0,3 (Interlocked.Increment) Master 0,1 (1,0 for int[2]) That lock free thing is really a nice thing. But if you read more about CPU cache, cache coherency, false sharing you can do even better. int[] Counters = new int[12]; // Cache line size is 64 bytes on my machine with an 8 way associative cache try for yourself e.g. 64 on more modern CPUs void Master() { var time = Stopwatch.StartNew(); Task t1 = Task.Factory.StartNew(ThreadWorkMethod_Master, 0); Task t2 = Task.Factory.StartNew(ThreadWorkMethod_Master, Counters.Length - 1); t1.Wait(); t2.Wait(); Counter = Counters[0] + Counters[Counters.Length - 1]; if (Counter != 2 * Increments) throw new Exception(String.Format("Hmm {0:N0} != {1:N0}", Counter, 2 * Increments)); Console.WriteLine("Master did take: {0:F1}s", time.Elapsed.TotalSeconds); } void ThreadWorkMethod_Master(object number) { int index = (int) number; for (int i = 0; i < Increments; i++) { Counters[index]++; } } The key insight here is to use for each core its own value. But if you simply use simply an integer array of two items, one for each core and add the items at the end you will be much slower than the lock free version (factor 3). Each CPU core has its own cache line size which is something in the range of 16-256 bytes. When you do access a value from one location the CPU does not only fetch one value from main memory but a complete cache line (e.g. 16 bytes). This means that you do not pay for the next 15 bytes when you access them. This can lead to dramatic performance improvements and non obvious code which is faster although it does have many more memory reads than another algorithm. So what have we done here? We have started with correct code but it was lacking knowledge how to use the .NET Base Class Libraries optimally. Then we did try to get fancy and used threads for the first time and failed. Our next try was better but it still had non obvious issues (lock object exposed to the outside). Knowledge has increased further and we have found a lock free version of our counter which is a nice and clean way which is a perfectly valid solution. The last example is only here to show you how you can get most out of threading by paying close attention to your used data structures and CPU cache coherency. Although we are working in a virtual execution environment in a high level language with automatic memory management it does pay off to know the details down to the assembly level. Only if you continue to learn and to dig deeper you can come up with solutions no one else was even considering. I have studied particle physics which does help at the digging deeper part. Have you ever tried to solve Quantum Chromodynamics equations? Compared to that the rest must be easy ;-). Although I am no longer working in the Science field I take pride in discovering non obvious things. This can be a very hard to find bug or a new way to restructure data to make something 10 times faster. Now I need to get some sleep ….

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• Linux arp cache timeout values

  - by Jak

  I'm trying to configure sane values for the Linux kernel arp cache timeout, but I can't find a detailed explanation as to how they work anywhere. Even the kernel.org documentation doesn't give a good explanation, I can only find recommended values to alleviate overflow. Here is an example of the values I have: net.ipv4.neigh.default.gc_thresh1 = 128 net.ipv4.neigh.default.gc_thresh2 = 512 net.ipv4.neigh.default.gc_thresh3 = 1024 Now, from what I've gathered so far: gc_thresh1 is the number of arp entries allowed before the garbage collector starts removing any entries at all. gc_thresh2 is the soft-limit, which is the number of entries allowed before the garbage collector actively removes arp entries. gc_thresh3 is the hard limit, where entries above this number are aggressively removed. Now, if I understand correctly, if the number of arp entries goes beyond gc_thresh1 but remains below gc_thresh2, the excess will be removed periodically with an interval set by gc_interval. My question is, if the number of entries goes beyond gc_thresh2 but below gc_thresh3, or if the number goes beyond gc_thresh3, how are the entries removed? In other words, what does "actively" and "aggressively" removed mean exactly? I assume it means they are removed more frequently than what is defined in gc_interval, but I can't find by how much.

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• How to efficiently serve massive sitemaps in django

  - by mlissner

  I have a site with about 150K pages in its sitemap. I'm using the sitemap index generator to make the sitemaps, but really, I need a way of caching it, because building the 150 sitemaps of 1,000 links each is brutal on my server.[1] I COULD cache each of these sitemap pages with memcached, which is what I'm using elsewhere on the site...however, this is so many sitemaps that it would completely fill memcached....so that doesn't work. What I think I need is a way to use the database as the cache for these, and to only generate them when there are changes to them (which as a result of the sitemap index means only changing the latest couple of sitemap pages, since the rest are always the same.)[2] But, as near as I can tell, I can only use one cache backend with django. How can I have these sitemaps ready for when Google comes-a-crawlin' without killing my database or memcached? Any thoughts? [1] I've limited it to 1,000 links per sitemap page because generating the max, 50,000 links, just wasn't happening. [2] for example, if I have sitemap.xml?page=1, page=2...sitemap.xml?page=50, I only really need to change sitemap.xml?page=50 until it is full with 1,000 links, then I can it pretty much forever, and focus on page 51 until it's full, cache it forever, etc.

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• VBScript: Disable caching of response from server to HTTP GET URL request

  - by Rob

  I want to turn off the cache used when a URL call to a server is made from VBScript running within an application on a Windows machine. What function/method/object do I use to do this? When the call is made for the first time, my Linux based Apache server returns a response back from the CGI Perl script that it is running. However, subsequent runs of the script seem to be using the same response as for the first time, so the data is being cached somewhere. My server logs confirm that the server is not being called in those subsequent times, only in the first time. This is what I am doing. I am using the following code from within a commercial application (don't wish to mention this application, probably not relevant to my problem): With CreateObject("MSXML2.XMLHTTP") .open "GET", "http://myserver/cgi-bin/nsr/nsr.cgi?aparam=1", False .send nsrresponse =.responseText End With Is there a function/method on the above object to turn off caching, or should I be calling a method/function to turn off the caching on a response object before making the URL? I looked here for a solution: http://msdn.microsoft.com/en-us/library/ms535874(VS.85).aspx - not quite helpful enough. And here: http://www.w3.org/TR/XMLHttpRequest/ - very unfriendly and hard to read. I am also trying to force not using the cache using http header settings and html document header meta data: Snippet of server-side Perl CGI script that returns the response back to the calling client, set expiry to 0. print $httpGetCGIRequest-header( -type = 'text/html', -expires = '+0s', ); Http header settings in response sent back to client: <html><head><meta http-equiv="CACHE-CONTROL" content="NO-CACHE"></head> <body> response message generated from server </body> </html> The above http header and html document head settings haven't worked, hence my question.

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