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• Java dropping half of UDP packets

  - by Andrew Klofas

  Greetings, I have a simple client/server setup. The server is in C and the client that is querying the server is Java. My problem is that, when I send bandwidth-intensive data over the connection, such as Video frames, it drops up to half the packets. I make sure that I properly fragment the udp packets on the server side (udp has a max payload length of 2^16). I verified that the server is sending the packets (printf the result of sendto()). But java doesn't seem to be getting half the data. Furthermore, when I switch to TCP, all the video frames get through but the latency starts to build up, adding several seconds delay after a few seconds of runtime. Is there anything obvious that I'm missing? I just can't seem to figure this out. Thanks, Andrew

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• synchronizing audio over a network

  - by sharkin

  I'm in startup of designing a client/server audio system which can stream audio arbitrarily over a network. One central server pumps out an audio stream and x number of clients receives the audio data and plays it. So far no magic needed and I have even got this scenario to work with VLC media player out of the box. However, the tricky part seems to be synchronizing the audio playback so that all clients are in audible synch (actual latency can be allowed as long as it is perceived to be in sync by a human listener). My question is if there's any known method or algorithm to use for these types of synchronization problems (video is probably solved the same way). My own initial thoughts centers around synchronizing clocks between physical machines and thereby creating a virtual "main timer" and somehow aligning audio data packets against it. Some products already solving the problem: http://www.sonos.com http://netchorus.com/ Any pointers are most welcome. Thanks. PS: This related question seem to have died long ago.

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• Can a CouchDB document update handler get an update conflict?

  - by jhs

  How likely is a revision conflict when using an update handler? Should I concern myself with conflict-handling code when writing a robust update function? As described in Document Update Handlers, CouchDB 0.10 and later allows on-demand server-side document modification. Update handlers can process non-JSON formats; but the other major features are these: An HTTP front-end to arbitrarily complex document modification code Similar code needn't be written for all possible clients—a DRY architecture Execution is faster and less likely to hit a revision conflict I am unclear about the third point. Executing locally, the update handler will run much faster and with lower latency. But in situations with high contention, that does not guarantee a successful update. Or does the update handler guarantee a successful update?

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• Image Drawing on UIView

  - by user1180261

  I'm trying to create an application where I can draw a lot of pictures at a specific point (determined for each image) on one view. I have a coordinates where I need draw a picture, width and height of it For example: I have 2 billion jpeg's images. for each images I have a specific origin point and size. In 1 second I need draw on view 20-50 images in specific point. I have already tryid solve that in the next way: UIGraphicsBeginImageContextWithOptions(self.previewScreen.bounds.size, YES, 0); [self.previewScreen.image drawAtPoint:CGPointMake(0, 0)]; [image drawAtPoint:CGPointMake(nRect.left, nRect.top)]; UIImage *imagew = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); [self.previewScreen setImage:imagew]; but in this solution I have a very big latency with displaying images and big CPU usage WBR Maxim Tartachnik

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• Pre-generating GUIDs for use in python?

  - by rjuiaa1

  I have a python program that needs to generate several guids and hand them back with some other data to a client over the network. It may be hit with a lot of requests in a short time period and I would like the latency to be as low as reasonably possible. Ideally, rather than generating new guids on the fly as the client waits for a response, I would rather be bulk-generating a list of guids in the background that is continually replenished so that I always have pre-generated ones ready to hand out. I am using the uuid module in python on linux. I understand that this is using the uuidd daemon to get uuids. Does uuidd already take care of pre-genreating uuids so that it always has some ready? From the documentation it appears that it does not. Is there some setting in python or with uuidd to get it to do this automatically? Is there a more elegant approach then manually creating a background thread in my program that maintains a list of uuids?

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• How to measure the time HTTP requests spend sitting in the accept-queue?

  - by David Jones

  I am using Apache2 on Ubuntu 9.10, and I am trying to tune my configuration for a web application to reduce latency of responses to HTTP requests. During a moderately heavy load on my small server, there are 24 apache2 processes handling requests. Additional requests get queued. Using "netstat", I see 24 connections are ESTABLISHED and 125 connections are TIME_WAIT. I am trying to figure out if that is considered a reasonable backlog. Most requests get serviced in a fraction of a second, so I am assuming requests move through the accept-queue fairly quickly, probably within 1 or 2 seconds, but I would like to be more certain. Can anyone recommend an easy way to measure the time an HTTP request sits in the accept-queue? The suggestions I have come across so far seem to start the clock after the apache2 worker accepts the connection. I'm trying to quantify the accept-queue delay before that. thanks in advance, David Jones

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• CruiseControl / NANT <copy> Task

  - by Striker

  We have a website with all the media (css/images) stored in a media folder. The media folder and it's 95 subdirectories contain about 400 total files. We have a Cruiscontrol project that monitors just the media directory for changes and when triggered copies those files to our integration server. Unfortunately, our integration server is at a remote location and so even when copying 2-3 files the NANT task is taking 4+ minutes. I believe the combination of the sheer number or directories/files and our network latency is causing the NANT task to run slow. I believe it is comparing the modified dates of both the local and remote copy of every file. I really want to speed this up and my initial thought was instead of trying to copy the whole media folder, can I get the list of file modifications from CruiseControl and specifically copy those files instead, saving the NANT task the work of having to compare them all for changes. Is there a way to do what I am asking or is there a better way to accomplish the same performance gains?

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• HTML Chrome Audit Specify Image Dimensions

  - by AKRamkumar

  I just started using the chrome developer tools for some basic html websites and I used the audit tool. I had two identical images, one with the height and width attribute, and one without. On the Resources section, both the latency and the download time were identical. However, the Audit showed Specify image dimensions (1) A width and height should be specified for all images in order to speed up page display. Does this actually help? And are there any other ways to speed up page time? This is only a splash page for the website I am building and as such it is only html, no css or javascript or anything. I have already compressed the images but I want to speed up load time even more. Is there a way?

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• WebSphere MQ/MQSeries - Possible to send a message to multiple queues with single call?

  - by Jeffrey White

  I'm queuing messages to a WebSphere MQ queue (NB: A point-to-point queue -- not a topic) using a stored procedure in my Oracle database. Is there a way to publish each message to multiple queues with a single call? What I would like is to find a solution that would incur zero additional latency on my database compared to sending the message to a single queue. Solutions that involve changing my WebSphere MQ settings are certainly welcome! What I had in mind was somehow creating a "clone" queue that got all the same messages as the original one, but I've been unable to locate anything like this in the documentation. Thanks, Jeff

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• Concise SSE and MMX instruction reference with latencies and throughput

  - by Joe

  I am trying to optimize some arithmetic by using the MMX and SSE instruction sets with inline assembly. However, I have been unable to find good references for the timings and usages of these enhanced instruction sets. Could you please help me find references that contain information about the throughput, latency, operands, and perhaps short descriptions of the instructions? So far, I have found: Intel Instruction References http://www.intel.com/Assets/PDF/manual/253666.pdf http://www.intel.com/Assets/PDF/manual/253667.pdf Intel Optimization Guide http://www.intel.com/Assets/PDF/manual/248966.pdf Timings of Integer Operations http://gmplib.org/~tege/x86-timing.pdf

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• C# Proxy using Sockets, how should I do this?

  - by Kin

  I'm writing a proxy using .NET and C#. I haven't done much Socket programming, and I am not sure the best way to go about it. What would be the best way to implement this? Should I use Synchronous Sockets, Asynchronous sockets? Please help! It must... Accept Connections from the client on two different ports, and be able to receive data on both ports at the same time. Connect to the server on two different ports, and be able to send data on both ports as the same time. Immediately connect to the server and start forwarding packets as soon as a client connection is made. Forward packets in the same order they were received. Be as low latency as possible. I don't need the ability for multiple clients to connect to the proxy, but it would be a nice feature if its easy to implement. Client --------- Proxy ------- Server ---|-----------------|----------------| Port <-------- Port <------- Port Port <-------- Port <------- Port

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• timing of reads from serial port on windows

  - by Marcin K

  I'm trying to implement a protocol over serial port on a windows(xp) machine. The problem is that message synchronization in the protocol is done via a gap in the messages, i.e., x millisecond gap between sent bytes signifies a new message. Now, I don't know if it is even possible to accurately detect this gap. I'm using win32/serport.h api to read in one of the many threads of our server. Data from the serial port gets buffered, so if there is enough (and there will be enough) latency in our software, I will get multiple messages from the port buffer in one sequence of reads. Is there a way of reading from the serial port, so that I would detect gaps in when particular bytes were received?

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• Commutative (operational transform) diffs for databases

  - by barrycarter

  What Unix program generates "diff"s between text files (or INSERT/UPDATE/DELETEs for databases) in such a way that the order that the "diff"s are applied in is irrelevant, and the result is the same regardless of order. Etherpad used to do something like this. Example (for a given document or database): % Adam makes a change X, then Bob makes a change Y, then Adam makes another change Z. % However, because of network latency, Adam sees the changes in this order: XZY, while Bob sees them in this order: YXZ. % However, the code/changes are written so that XYZ and YXZ yield the same result. Note: ideally, this can be done without having to do X/Y/Z inverse at any point. I have read http://stackoverflow.com/questions/2043165/operational-transformation-library but I'm not sure this really does what I want.

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• "conveyor belt" cache architecture

  - by Andrew Matthews

  I'm producing an application with a few peculiar internal communication characteristics that make the usual suspects for data storage and transport (Qs and RDBMSs) ill-fitted. I'm wondering whether there is a product out there that matches the following characteristics: all data put into it is peristent all reads are delivered out of memory data is universally available data lives where it is most needed data is versioned (nice to have) updates are transactional (I'd like ACID characteristics) data is potentially replicated, but always in sync works on windows is based on or has bindings for .NET is really fast is really robust is redundant is scalable I'm looking at things like Microsoft codename "Velocity", but I am not sure whether it fits all of the above characteristics. Likewise, Memcached is not a perfect fit either. The current version of this app opts for an RDBMS with a signaling system for inter-system sync, but latency is too high and versioning of the DB is a pain. I need all the robustness, but with none of the trade-offs.

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• Copying Some from a PostgreSQL Server to Another

  - by whollychao

  I am in need of an application that can periodically transmit select rows from a PostgreSQL database across a network to a second PostgreSQL server. Typically these will be the most recent row added, pulled and transmitted every 10-30 seconds. The primary servers run in a MS Windows environment with a high-latency, and occasionally intermittent, network connection. Therefore, any application would have to be tolerant of this and ideally automatically reconnect / resend data that could not be transmitted. Due to the environment and the requirements, a full-blown replication package would be unnecessary. I appreciate any help anyone has with this problem.

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• C# Proxy, what is the best way to do this?

  - by Kin

  I'm writing a proxy using .NET and C#. It has a couple of functions that it needs to fulfill. I haven't done much Socket programming, and I am not sure the best way to go about it. Should I use Synchronous Sockets, Asynchronous sockets? Please help! It must... Accept Connections from the client on two different ports, and be able to receive data on both ports at the same time. When a connection is made on a port, it must immediately connect to the server, and start sending data as it receives it from the client to the server. Packets must be forwarded in the order they are received, exactly as they were received. It needs to be as low latency as possible. I don't need the ability for multiple clients to use the proxy, but it would be a nice feature if its easy to implement.

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• What is the chance a CouchDB document update handler will get a revision conflict?

  - by jhs

  How likely is a revision conflict when using an update handler? Should I concern myself with conflict-handling code when writing a robust update function? As described in Document Update Handlers, CouchDB 0.10 and later allows on-demand server-side document modification. Update handlers can process non-JSON formats; but the other major features are these: An HTTP front-end to arbitrarily complex document modification code Similar code needn't be written for all possible clients—a DRY architecture Execution is faster and less likely to hit a revision conflict I am unclear about the third point. Executing locally, the update handler will run much faster and with lower latency. But in situations with high contention, that does not guarantee a successful update. Or does the update handler guarantee a successful update?

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• About data size filled in the buffer

  - by Bohan Lu

  I need low-latency audio in my project, and I know Android 2.3 supports OpenSL ES. I have read documents and sample code and I decide to use Android simple buffer queue to do the play and record. I now try to write a simple application to do the test. However, I have some questions about recording. If I set the recorder stop when it is recording, how do I know the exact number of bytes filled in the last buffer if it is not filled up ? In 1.1 version, the callback function has some parameters about buffer and its filled data, but there is no such parameters in version 1.0.1. Is there any way to get this information ? Any suggestion would be greatly appreciated !

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• Is there simple way to play an rtp video/audio stream in WPF?

  - by Robin

  I need to create a WPF control that will play an rtp stream with the requirement that the latency needs to be as low as possible. I've looked at the following two projects: http://vlcdotnet.codeplex.com/ http://wpfmediakit.codeplex.com/ As far as I know, I can't use VLC because we're shipping a commercial application with a more restrictive license than GPL (i.e. we can't ship our source). Wpf media kit is nice, but I can't seem to find a good/free rtp directshow source filter and I wanted to ask if there is a simpler solution out there that I'm missing before I jump into writing my own. Any ideas?

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• Should I use a hosted version of JQuery? Which one?

  - by ataylor

  Should I use a local copy of jquery, or should I link to a copy provided by Google or Microsoft? I'm primarily concerned about speed. I've heard that just pulling content from other domains can have performance advantages related to how browsers limit connections. In particular, has anyone benchmarked the speed and latency of Google vs. Microsoft vs. local? Also, do I have to agree to any conditions or licenses to link from a third-party?

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• Best approach to send data from a server to an Android device

  - by ElectricDialect

  I am developing an Android app that needs to communicate bi-directionally with a server. By that, I mean either the server or the device can send a message at any time, with an arbitrary amount of time in between messages. Sending data from the device to the server is a common and I think well understood task, but I'm not as sure what the best approach is to go in the opposite direction from the server to the device. I think having the device periodically poll the server may be a bad idea due to latency and the drain on the battery, but I'd be willing to consider this option. My plan at the moment is to send text messages from the server via an email-to-SMS bridge, and to have my app run a service to receive and handle these messages. The question I have is if there are any best practices for this scenario, and if using text messages has some downsides that I have failed to consider. For the sake of this question, I want to assume that users have an unlimited text data plan, so paying per text won't be an issue.

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• Selenium WebDriver works but SLOW (Java)

  - by Chris

  Code: WebDriver driver = new FirefoxDriver(); driver.get("http://www.cnn.com"); File scrFile = ((TakesScreenshot)driver).getScreenshotAs(OutputType.FILE); FileUtils.copyFile(scrFile, new File("c:\\test\\screenshot.png")); I am using Selenium WebDriver to take a screenshot of webpages. It runs great. However, from the time I hit run in eclipse to the time the screenshot shows up in my local drive is 7-10 seconds. Most of the latency seems to be launching Firefox. How can I speed up this process? Is there a way that I can use an already opened Firefox browser to save on opening a new one? Is this code somehow heavy? Details- Tried on CentOS box and Win7 box both using eclipse. myspeedtest.net shows 22Mbps down and 1 Mbps up.

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• What Use are Threads Outside of Parallel Problems on MultiCore Systesm?

  - by Robert S. Barnes

  Threads make the design, implementation and debugging of a program significantly more difficult. Yet many people seem to think that every task in a program that can be threaded should be threaded, even on a single core system. I can understand threading something like an MPEG2 decoder that's going to run on a multicore cpu ( which I've done ), but what can justify the significant development costs threading entails when you're talking about a single core system or even a multicore system if your task doesn't gain significant performance from a parallel implementation? Or more succinctly, what kinds of non-performance related problems justify threading? Edit Well I just ran across one instance that's not CPU limited but threads make a big difference: TCP, HTTP and the Multi-Threading Sweet Spot Multiple threads are pretty useful when trying to max out your bandwidth to another peer over a high latency network connection. Non-blocking I/O would use significantly less local CPU resources, but would be much more difficult to design and implement.

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• MySQL Cluster data nodes - slow SELECTs

  - by Boyan Georgiev

  Hi to all. First off, I'm new to MySQL Cluster. This is my pain: I've managed to setup a MySQL Cluster with two data nodes, two SQL nodes and one management server. Everything works pretty well, except the following: my data nodes are spread across an intranet link which incurs latency into communications between the data nodes. Apparently, due to MySQL Cluster's internal partitioning schemes, when my PHP application pulls data from the cluster via SELECT queries, parts of the data are pulled from both data nodes. This makes the page appear onscreen REALLY slowly. If I bring one data node offline, the data can only be pulled from that single remaining data node, and thus, the final result (HTML output) appears on the screen in a very timely fashion. So, my question is this: can the data nodes/cluster be told to pull data from partitions stored only on a particular data node?

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• UDP + total order, non-reliable

  - by disown

  I'm trying to find a version of UDP which just alleviates the restriction of a maximum size of the message sent. I don't care about reliability or partial retransmission, if all chunks arrive I want the message to be assembled from the chunks in sending order and delivered to the listening app. If one or more chunks are missing I would just like to discard the message. The goal is to have a low-latency notification mechanism about real time data, but with the added support for bigger messages than what would fit in an IP datagram. I would like the protocol to be one way only, and not have long connection setup times. An optional feature to be able to respond to a received message wouldn't hurt (a concept of an unreliable connection), but is not necessary.

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