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Re: Thread scaling in Windows

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Marcel Müller <news.5.maazl@spamgourmet.org> wrote in
news:51a8ab52$0$6567$9b4e6d93@newsspool4.arcor-online.net: 

> On 31.05.13 00.52, Paavo Helde wrote:
>>> Most likely you enter the mutexes very often during calculation.
>>> This 
>> is
>>> bad practice. If you want to run in parallel, then run in parallel
>>> and do not talk to each other over an over.
>>
>> Mutexes are indeed entered quite often, but most of the cases there
>> should be no contention and the mutex is locked for a very short
>> time, 
> 
> This does not care. Think of that: entering a mutex requires to 
> synchronize all cores with memory. Even at lowest level one clock
> cycle of 1GHz (very few nowadays) is only enough for synchronizing
> over a distance of about 10 cm, simply because the speed of the
> electric fields (about 70% of the speed of light). Enough to reach the
> memory chips, but no more. Switching times of the electronics will
> additionally reduce the speed. So even at assembly level, e.g. with
> lock-free algorithms, synchronization (= memory barriers) are
> expensive nowadays. 
> 
>> like updating a shared object reference count by a smartpointer.
> 
> This is really expensive! Not only because of the mutex. Writing to a 
> shared memory location from different cores also invalidates the
> related cache content of all cores except for the one currently
> writing. So cache misses are very likely to occur after that. The
> problem is called cache line hopping, and you simply should not do
> that. 

Right, this is something I have vaguely suspected myself. Trying to get 
rid of them. But this does not explain the Linux-Windows difference, does 
it?

> 
> But if so, why do you write to a shared smart pointer all the time?

We have two kinds of refcounted smartpointers - for single-threaded and 
shared objects. Mostly the single-threaded objects are used, otherwise 
there would be hundreds of times more mutex locks. Shared objects are 
accessed only occasionally, but this still makes up a lot as it seems. In 
principle it should suffice to have only one shared object smartpointer 
in a thread and use C++ references for passing it around in the thread.

Thanks for the answers!
Paavo

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Thread

Thread scaling in Windows Paavo Helde <myfirstname@osa.pri.ee> - 2013-05-29 12:31 -0500
  Re: Thread scaling in Windows Robert Miles <robertmilesxyz@gmail.com> - 2013-05-29 17:26 -0700
    Re: Thread scaling in Windows Paavo Helde <myfirstname@osa.pri.ee> - 2013-05-30 16:11 -0500
      Re: Thread scaling in Windows Gerald Breuer <Gerald.Breuer@googlemail.com> - 2013-05-31 03:44 +0200
        Re: Thread scaling in Windows Paavo Helde <myfirstname@osa.pri.ee> - 2013-05-31 00:29 -0500
  Re: Thread scaling in Windows Marcel Müller <news.5.maazl@spamgourmet.org> - 2013-05-30 23:28 +0200
    Re: Thread scaling in Windows Paavo Helde <myfirstname@osa.pri.ee> - 2013-05-30 17:52 -0500
      Re: Thread scaling in Windows Melzzzzz <mel@zzzzz.com> - 2013-05-31 04:18 +0200
      Re: Thread scaling in Windows Robert Wessel <robertwessel2@yahoo.com> - 2013-05-31 01:11 -0500
        Re: Thread scaling in Windows Paavo Helde <myfirstname@osa.pri.ee> - 2013-06-01 12:15 -0500
          Re: Thread scaling in Windows Paavo Helde <myfirstname@osa.pri.ee> - 2013-06-01 16:52 -0500
            Re: Thread scaling in Windows [SOLVED] Paavo Helde <myfirstname@osa.pri.ee> - 2013-06-03 12:00 -0500
              Re: Thread scaling in Windows [SOLVED] Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2013-06-04 08:16 +0000
              Re: Thread scaling in Windows [SOLVED] Noob <root@127.0.0.1> - 2013-06-04 10:25 +0200
      Re: Thread scaling in Windows Marcel Müller <news.5.maazl@spamgourmet.org> - 2013-05-31 15:53 +0200
        Re: Thread scaling in Windows Paavo Helde <myfirstname@osa.pri.ee> - 2013-05-31 12:57 -0500
          Re: Thread scaling in Windows Marcel Müller <news.5.maazl@spamgourmet.org> - 2013-05-31 22:59 +0200
            Re: Thread scaling in Windows Paavo Helde <myfirstname@osa.pri.ee> - 2013-06-01 03:06 -0500
              Re: Thread scaling in Windows Marcel Müller <news.5.maazl@spamgourmet.org> - 2013-06-01 15:20 +0200
                Re: Thread scaling in Windows Drazen Kacar <dave@fly.srk.fer.hr> - 2013-06-02 06:35 +0000
            Re: Thread scaling in Windows Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2013-06-01 12:34 +0000
              Re: Thread scaling in Windows Marcel Müller <news.5.maazl@spamgourmet.org> - 2013-06-01 15:09 +0200
                Re: Thread scaling in Windows Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2013-06-01 17:12 +0000

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