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Search found 413 results on 17 pages for 'atomic clocks'.

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  • Are C++ Reads and Writes of an int atomic

    - by theschmitzer
    I have two threads, one updating an int and one reading it. This value is a statistic where the order of the read and write is irrelevant. My question is, do I need to synchronize access to this multi-byte value anyway? Or, put another way, can part of the write be complete and get interrupted, and then the read happen. For example, think of value = ox0000FFFF increment value to 0x00010000 Is there a time where the value looks like 0x0001FFFF that I should be worried about? Certainly the larger the type, the more possible something like this is I've always synchronized these types of accesses, but was curious what the community thought.

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  • Are Triggers Based On Queries Atomic?

    - by David
    I have a table that has a Sequence number. This sequence number will change and referencing the auto number will not work. I fear that the values of the trigger will collide. If two transactions read at the same time. I have ran simulated tests on 3 connections @ ~1 million records each and no collisions. CREATE TABLE `aut` ( `au_id` int(10) NOT NULL AUTO_INCREMENT, `au_control` int(10) DEFAULT NULL, `au_name` varchar(50) DEFAULT NULL, `did` int(10) DEFAULT NULL, PRIMARY KEY (`au_id`), KEY `Did` (`did`) ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=latin1 TRIGGER `binc_control` BEFORE INSERT ON `aut` FOR EACH ROW BEGIN SET NEW.AU_CONTROL = (SELECT COUNT(*)+1 FROM aut WHERE did = NEW.did); END;

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  • iPhone file corruption

    - by sfider
    Is it possible (on iPhone/iPod Touch) for a file written like this: if (FILE* file = fopen(filename, "wb")) { fwrite(buf, buf_size, 1, file); fclose(file); } to get corrupted, e.g. when app is forced to terminate? From what I know fwrite should be an atomic operation, so when I write whole file with one instruction no corruption should occure. I could not find any information on the net that would say otherwise.

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  • Guaranteeing ACID properties for InnoDB databases.

    - by plinehan
    What steps must one take to ensure that an otherwise defaultly-configured InnoDB server is truly ACID compliant? The InnoDB configuration page mentions that the hardware itself must be configured to honor fsync calls, i.e. disable any write-back caches. This page mentions some other concerns, but may be conflating the binary log and the InnoDB log, and may be a bit out of date regarding default settings for MySQL 5.x. Upon reading the binary log document page it would seem that the "sync_binlog=1" setting is not required for ACID properties in general, only for ACID properties vis a vis point-in-time recovery and replication. So, is disabling write-back disk caching sufficient, or are there other settings that must be tweaked?

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  • How do you implement Software Transactional Memory?

    - by Joseph Garvin
    In terms of actual low level atomic instructions and memory fences (I assume they're used), how do you implement STM? The part that's mysterious to me is that given some arbitrary chunk of code, you need a way to go back afterward and determine if the values used in each step were valid. How do you do that, and how do you do it efficiently? This would also seem to suggest that just like any other 'locking' solution you want to keep your critical sections as small as possible (to decrease the probability of a conflict), am I right? Also, can STM simply detect "another thread entered this area while the computation was executing, therefore the computation is invalid" or can it actually detect whether clobbered values were used (and thus by luck sometimes two threads may execute the same critical section simultaneously without need for rollback)?

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  • Volatile or synchronized for primitive type?

    - by DKSRathore
    In java, assignment is atomic if the size of the variable is less that or equal to 32 bits but is not if more than 32 bits. What(volatile/synchronized) would be more efficient to use in case of double or long assignment. like, volatile double x = y; synchronized is not applicable with primitive argument. How do i use synchronized in this case. Of course I don't want to lock my class. so this should not be used.

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  • How do you implement Software Transactional Memory?

    - by Joseph Garvin
    In terms of actual low level atomic instructions and memory fences (I assume they're used), how do you implement STM? The part that's mysterious to me is that given some arbitrary chunk of code, you need a way to go back afterward and determine if the values used in each step were valid. How do you do that, and how do you do it efficiently? This would also seem to suggest that just like any other 'locking' solution you want to keep your critical sections as small as possible (to decrease the probability of a conflict), am I right? Also, can STM simply detect "another thread entered this area while the computation was executing, therefore the computation is invalid" or can it actually detect whether clobbered values were used (and thus by luck sometimes two threads may execute the same critical section simultaneously without need for rollback)?

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  • Is it possible to store pointers in shared memory without using offsets?

    - by Joseph Garvin
    When using shared memory, each process may mmap the shared region into a different area of their address space. This means that when storing pointers within the shared region, you need to store them as offsets of the start of the shared region. Unfortunately, this complicates use of atomic instructions (e.g. if you're trying to write a lock free algorithm). For example, say you have a bunch of reference counted nodes in shared memory, created by a single writer. The writer periodically atomically updates a pointer 'p' to point to a valid node with positive reference count. Readers want to atomically write to 'p' because it points to the beginning of a node (a struct) whose first element is a reference count. Since p always points to a valid node, incrementing the ref count is safe, and makes it safe to dereference 'p' and access other members. However, this all only works when everything is in the same address space. If the nodes and the 'p' pointer are stored in shared memory, then clients suffer a race condition: x = read p y = x + offset Increment refcount at y During step 2, p may change and x may no longer point to a valid node. The only workaround I can think of is somehow forcing all processes to agree on where to map the shared memory, so that real pointers rather than offsets can be stored in the mmap'd region. Is there any way to do that? I see MAP_FIXED in the mmap documentation, but I don't know how I could pick an address that would be safe.

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  • Parallel version of loop not faster than serial version

    - by Il-Bhima
    I'm writing a program in C++ to perform a simulation of particular system. For each timestep, the biggest part of the execution is taking up by a single loop. Fortunately this is embarassingly parallel, so I decided to use Boost Threads to parallelize it (I'm running on a 2 core machine). I would expect at speedup close to 2 times the serial version, since there is no locking. However I am finding that there is no speedup at all. I implemented the parallel version of the loop as follows: Wake up the two threads (they are blocked on a barrier). Each thread then performs the following: Atomically fetch and increment a global counter. Retrieve the particle with that index. Perform the computation on that particle, storing the result in a separate array Wait on a job finished barrier The main thread waits on the job finished barrier. I used this approach since it should provide good load balancing (since each computation may take differing amounts of time). I am really curious as to what could possibly cause this slowdown. I always read that atomic variables are fast, but now I'm starting to wonder whether they have their performance costs. If anybody has some ideas what to look for or any hints I would really appreciate it. I've been bashing my head on it for a week, and profiling has not revealed much.

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  • PHP rewrite to included file - is this a valid script?

    - by Poni
    Hi all! I've made this question: http://stackoverflow.com/questions/2921469/php-mutual-exclusion-mutex As said there, I want several sources to send their stats once in a while, and these stats will be showed at the website's main page. My problem is that I want this to be done in an atomic manner, so no update of the stats will overlap another one running in the background. Now, I came up with this solution and I want you PHP experts to judge it. stats.php <?php define("my_counter", 12); ?> index.php <?php include "stats.php"; echo constant("my_counter"); ?> update.php <?php $old_error_reporting = error_reporting(0); include "stats.php"; define("my_stats_template",' <?php define("my_counter", %d); ?> '); $fd = fopen("stats.php", "w+"); if($fd) { if (flock($fd, LOCK_EX)) { $my_counter = 0; try { $my_counter = constant("my_counter"); } catch(Exception $e) { } $my_counter++; $new_stats = sprintf(constant("my_stats_template"), $my_counter); echo "Counter should stand at $my_counter"; fwrite($fd, $new_stats); } flock($fd, LOCK_UN); fclose($fd); } error_reporting($old_error_reporting); ?> Several clients will call the "update.php" file once every 60sec each. The "index.php" is going to use the "stats.php" file all the time as you can see. What's your opinion?

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  • Do I need to using locking against integers in c++ threads

    - by Shane MacLaughlin
    The title says it all really. If I am accessing a single integer type (e.g. long, int, bool, etc...) in multiple threads, do I need to use a synchronisation mechanism such as a mutex to lock them. My understanding is that as atomic types, I don't need to lock access to a single thread, but I see a lot of code out there that does use locking. Profiling such code shows that there is a significant performance hit for using locks, so I'd rather not. So if the item I'm accessing corresponds to a bus width integer (e.g. 4 bytes on a 32 bit processor) do I need to lock access to it when it is being used across multiple threads? Put another way, if thread A is writing to integer variable X at the same time as thread B is reading from the same variable, is it possible that thread B could end up a few bytes of the previous value mixed in with a few bytes of the value being written? Is this architecture dependent, e.g. ok for 4 byte integers on 32 bit systems but unsafe on 8 byte integers on 64 bit systems? Edit: Just saw this related post which helps a fair bit.

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  • PHP rewrite an included file - is this a valid script?

    - by Poni
    Hi all! I've made this question: http://stackoverflow.com/questions/2921469/php-mutual-exclusion-mutex As said there, I want several sources to send their stats once in a while, and these stats will be showed at the website's main page. My problem is that I want this to be done in an atomic manner, so no update of the stats will overlap another one running in the background. Now, I came up with this solution and I want you PHP experts to judge it. stats.php <?php define("my_counter", 12); ?> index.php <?php include "stats.php"; echo constant("my_counter"); ?> update.php <?php $old_error_reporting = error_reporting(0); include "stats.php"; define("my_stats_template",' <?php define("my_counter", %d); ?> '); $fd = fopen("stats.php", "w+"); if($fd) { if (flock($fd, LOCK_EX)) { $my_counter = 0; try { $my_counter = constant("my_counter"); } catch(Exception $e) { } $my_counter++; $new_stats = sprintf(constant("my_stats_template"), $my_counter); echo "Counter should stand at $my_counter"; fwrite($fd, $new_stats); } flock($fd, LOCK_UN); fclose($fd); } error_reporting($old_error_reporting); ?> Several clients will call the "update.php" file once every 60sec each. The "index.php" is going to use the "stats.php" file all the time as you can see. What's your opinion?

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  • CUDA: accumulate data into a large histogram of floats

    - by shoosh
    I'm trying to think of a way to implement the following algorithm using CUDA: Working on a large volume of voxels, for each voxel I calculate an index i and a value c. after the calculation I need to perform histogram[i] += c c is a float value and the histogram can have up to 15,000 bins. I'm looking for a way to implement this efficiently using CUDA. The first obvious problem is that with compute capabilities 1.3 which is what I'm using I can't even do an atomicAdd() of floats so how can I accumulate anything reliably? This example by nVidia does something somewhat simpler. The histograms are saved in the shared memory (which I can't do due to its size) and it only accumulates integers. Can this approach be generalized to my case?

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  • How does _mm_mwait works?

    - by osgx
    Hello How does _mm_mwait from pmmintrin.h works? (I mean not the asm for it, but action and how this action is taken in NUMA systems. The store monitoring is easy to implement only on bus-based SMP systems with snooping of bus.) What processors does implement it? Is it used in some spinlocks?

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  • how to atomically claim a row or resource using UPDATE in mysql

    - by Igor
    i have a table of resources (lets say cars) which i want to claim atomically. if there's a limit of one resource per one user, i can do the following trick: UPDATE cars SET user = 'bob' WHERE user IS NULL LIMIT 1 SELECT * FROM cars WHERE user IS bob that way, i claim the resource atomically and then i can see which row i just claimed. this doesn't work when 'bob' can claim multiple cars. i realize i can get a list of cars already claimed by bob, claim another one, and then SELECT again to see what's changed, but that feels hackish. What I'm wondering is, is there some way to see which rows i just updated with my last UPDATE? failing that, is there some other trick to atomically claiming a row? i really want to avoid using SERIALIZABLE isolation level. If I do something like this: 1 SELECT id FROM cars WHERE user IS NULL 2 <here, my PHP or whatever picks a car id> 3 UPDATE cars SET user = 'bob' WHERE id = <the one i picked> would REPEATABLE READ be sufficient here? in other words, could i be guaranteed that some other transactions won't claim the row my software has picked during step 2?

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  • Where to find __sync_add_and_fetch_8?

    - by Viet
    I got errors when trying to use __sync_add_and_fetch: test8.cpp:(.text+0x90e): undefined reference to `__sync_add_and_fetch_8' collect2: ld returned 1 exit status Please kindly advise how to rectify this. Specs: GCC/G++: 4.4.1 GNU/Linux 2.6.32 SMP i686 Many thanks!

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  • Is there a good way to execute MySQL statements atomically via JDBC?

    - by javanix
    Suppose I have a table that contains valid data. I would like to modify this data in some way, but I'd like to make sure that if any errors occur with the modification, the table isn't changed and the method returns something to that effect. For instance, (this is kind of a dumb example, but it illustrates the point so bear with me) suppose I want to edit all the entries in a "name" column so that they are properly capitalized. For some reason, I want either ALL of the names to have proper capitalization, or NONE of them to have proper capitalization (and the starting state of the table is that NONE of them do). Is there an already-implemented way to run a batch update on the table and be assured that, if any one of the updates fails, all changes are rolled back and the table remains unchanged? I can think of a few ways to do this by hand (though suggestions are welcomed), but it'd be nice if there was some method I could use that would function this way. I looked at the java.sql.statement.executeBatch() command, but I'm not convinced by the documentation that my table wouldn't be changed if it failed in some manner.

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  • python iterators and thread-safety

    - by Igor
    I have a class which is being operated on by two functions. One function creates a list of widgets and writes it into the class: def updateWidgets(self): widgets = self.generateWidgetList() self.widgets = widgets the other function deals with the widgets in some way: def workOnWidgets(self): for widget in self.widgets: self.workOnWidget(widget) each of these functions runs in it's own thread. the question is, what happens if the updateWidgets() thread executes while the workOnWidgets() thread is running? I am assuming that the iterator created as part of the for...in loop will keep some kind of reference to the old self.widgets object? So I will finish iterating over the old list... but I'd love to know for sure.

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  • Django: How can I protect against concurrent modification of data base entries

    - by Ber
    If there a way to protect against concurrent modifications of the same data base entry by two or more users? It would be acceptable to show an error message to the user performing the second commit/save operation, but data should not be silently overwritten. I think locking the entry is not an option, as a user might use the "Back" button or simply close his browser, leaving the lock for ever.

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