Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c++ > #81570

Re: Some help needed

Show all headers | View raw

Am 25.09.2021 um 18:51 schrieb Paavo Helde:
> 25.09.2021 17:41 Bonita Montero kirjutas:
>> I've developed a monitor-object like that of Java for C++ with some
>> improvements. The major improvement is that there's not only a spin
>> -loop for locking and unlocking but also for waiting on an event. In
>> this case you don't have to lock the mutex but supply a predicate on
>> a wait_poll-function and the code repeatedly tries to lock the mutex
>> polling and if it can lock the mutex it calls the predicate which
>> returns (or moves) a pair of a bool and the result-type.
>> Waiting to for a semaphore and or a event-object (Win32) in the kernel
>> can easily take from 1.000 to 10.000 clock-cylces even when the call
>> immediately returns because the semaphore or event has been set before.
>> So there has to be a spin count with a reasonable relationship to this
>> waiting-inteval, f.e. spinning one tenth of the minimum interval being
>> spent in the kernel.
>> With my monitor-object I've taken the spincount recalculation-algorithm
>> from the glibc. And I'm also using the PAUSE-instruction. But I think
>> that the glibc induces to heavy cacheline-flipping by re-loading the
>> mutex-flags immediately after a single PAUSE-instruction. So I decided
>> to loop PAUSE several times and to take less spinning iterations there-
>> fore.
>> To get a reasonable number of PAUSE-spinnings I need the time PAUSE
>> takes on different processors. On my CPU PAUSE halts the pipe only
>> for about for 0,78 nanoseconds, which is about 3,25 clock-cycles in
>> average. I've written a short progam tha repeatedly PAUSEs and takes
>> the aveage time. I want to encourage you to compile the code on your
>> machine and give me the PAUSE-timing it outputs here.
>>
>> This is the code:
>>
>> #include <iostream>
>> #include <chrono>
>> #include <cstddef>
>> #include <cstdint>
>> #include <immintrin.h>
>>
>> using namespace std;
>> using namespace chrono;
>>
>> int main( int argc, char **argv )
>> {
>>      static uint64_t const PAUSE_ROUNDS = 1'000'000'000;
>>      auto start = high_resolution_clock::now();
>>      for( uint64_t i = PAUSE_ROUNDS; i; --i )
>>          _mm_pause();
>>      double ns = (int64_t)duration_cast<nanoseconds>( 
>> high_resolution_clock::now() - start ).count() / (double)PAUSE_ROUNDS;
>>      cout << ns << endl;
>> }
> 
> On my computer this outputs:
> 
> 34.9194

That seems much more reasonable to me than the < 4 clock cycles on my
PC (Ryzen Threadripper 3990X).

Back to comp.lang.c++ | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Some help needed Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-25 16:41 +0200
  Re: Some help needed Paavo Helde <myfirstname@osa.pri.ee> - 2021-09-25 19:51 +0300
    Re: Some help needed Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-25 19:01 +0200
    Re: Some help needed Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-25 17:20 +0000
  Re: Some help needed Bo Persson <bo@bo-persson.se> - 2021-09-25 19:05 +0200
    Re: Some help needed Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-25 19:14 +0200
    Re: Some help needed Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-25 17:33 +0000
  Re: Some help needed Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-25 17:15 +0000
    Re: Some help needed Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-25 19:16 +0200
      Re: Some help needed Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-25 17:29 +0000
  Re: Some help needed Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-26 07:26 +0200
    Re: Some help needed Branimir Maksimovic <branimir.maksimovic@gmail.com> - 2021-09-26 06:56 +0000
    Re: Some help needed - further help Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-27 17:35 +0200
      Re: Some help needed - further help Bo Persson <bo@bo-persson.se> - 2021-09-27 19:29 +0200

csiph-web