Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Like all data structures in standard library, methods of <code>vector</code> are reentrant, but not thread-safe. That means different instances can be accessed by multiple threads independently, but each instance may only be accessed by one thread at a time and you have to ensure it. But since you have separate vector for each thread, that's not your problem.</p> <p>What is probably your problem is the <code>printf</code>. <code>printf</code> is thread-safe, meaning you can call it from any number of threads at the same time, but at the cost of being wrapped in mutual exclusion internally.</p> <p>Majority of work in the threaded part of your program is done inside <code>printf</code>. So what probably happens is that all the threads are started and quickly get to the <code>printf</code>, where all but the first will stop. When the printf finishes and releases the mutex, system considers scheduling the threads that were waiting for it. It probably does, so rather slow context switch happens. And repeats after every <code>printf</code>.</p> <p>How exactly it happens depends on which actual locking primitive is being used, which depends on your operating system and standard library versions. The system should each time wake up only the next sleeper, but many implementations actually wake up all of them. So in addition to the <code>printf</code>s being executed in mostly round-robin fashion, incurring one context switch for each, there may be quite a few additional spurious wake-ups in which the thread just finds the lock is held and goes back to sleep.</p> <p>So the lesson from this is that threads don't make things automagically faster. They only help when:</p> <ul> <li>The thread spends most of it's time doing blocking system calls. In things like network servers the threads wait for data from the socket, than from data for response to come from disk and finally for network to accept the response. In such cases, having many threads helps as long as they are mostly independent.</li> <li>There is just so many threads as there are CPU threads. Currently the usual number is 4 (either quad-core or dual-core with hyper-threading). More threads can't physically run in parallel, so they provide no gain and incur a bit of overhead. 16 threads is thus overkill.</li> </ul> <p>And they never help when they all manipulate the same objects, so they end up spending most of the time waiting for locks anyway. In addition to any of your own objects that you lock, keep in mind that input and output file handles have to be internally locked as well.</p> <p>Memory allocation also needs to internally synchronize between threads, but modern allocators have separate pools for threads to avoid much of it; if the default allocator proves to be too slow with many threads, there are some specialized ones you can use.</p>
    singulars
    1. This table or related slice is empty.
    1. POC: performance of pthread, low than single thrad
      singulars
      1. PTQuestion
    1. PTAnswer
    1. This table or related slice is empty.
    1. USJan Hudec
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POC: performance of pthread, low than single thrad
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. This table or related slice is empty.
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload