Re: Cygwin multithreading performance

[Kacper Michajlow <kasper93@gmail.com>]

2015-12-06 9:02 GMT+01:00 Mark Geisert <mark@maxrnd.com>:
> Kacper Michajlow wrote:
>>
>> 2015-12-05 23:40 GMT+01:00 Mark Geisert <mark@maxrnd.com>:
>>>
>>> It looks like we're going to have to compare actual pthread_mutex_lock()
>>> implementations.  Inspecting source is nice but I don't want to be
>>> chasing a
>>> mirage so I really hope there's a pthread_mutex_lock() function inside
>>> the
>>> MinGW git you are running.  gdb could easily answer that question.  Could
>>> you please do an 'info func pthread_mutex_lock' after starting MinGW git
>>> under MinGW gdb with a breakpoint at main() (so libraries are loaded).
>
> [...]
>>
>> Hmm, thinking about it mingw doesn't have pthread implementation or
>> any wrapper for it. If someone needs pthread they would probably go
>> for pthreads-w32 implementation.
>>
>> I started to wonder because I don't recall git would need pthreads to
>> compile on Windows. And indeed they have a wrapper for Windows API...
>> https://github.com/git/git/blob/master/compat/win32/pthread.h
>> https://github.com/git/git/blob/master/compat/win32/pthread.c
>
>
> OK, so git has its own pthread_mutex_lock/unlock ops which map to very
> light-weight critical section operations.
>
>> Though it is not really a matter that "native" git build is fast and
>> all, but that Cygwin's one really struggles if it comes to MT workload.
>
>
> In the worst cases I see using your testcase, about half the time the
> busiest locks are processed within 1 usec but there's a spectrum of longer
> latencies for the other half of the time.  I don't know (yet) if that can be
> improved in Cygwin's more general implementation but at least the matter has
> now been brought to our attention :).
,
Yes, I can imagine, git's objects are very small so threading overhead
is very noticeable.

>> And this not only issue with git unfortunately. Download speeds are
>> also limited on Cygwin. I know POSIX compatibility layers comes with a
>> price but I would love to see improvements in those areas.
>> Cygwin:
>> Receiving objects: 100% (230458/230458), 78.41 MiB | 1.53 MiB/s, done.
>> "native" git:
>> Receiving objects: 100% (230458/230458), 78.41 MiB | 18.54 MiB/s, done.
>
>
> You're asserting this additional testcase has the same cause.  What is
> telling you that?  And FTR what is the git command you are issuing?  I can
> then do the lock latency analysis on this new testcase if warranted.

No, sorry, I mixed different things. It is just that I'm ruining both
git build lately and I wanted to share another issue before I forget
about it.

This was git clone command for some random repository from github.
There is a lot factors at hand here but the fact is with cygwin speed
is capped on 1.5MB/s and this is reproducible. This is probably also
related to the fact that git operates on large amount small object.
But this time it is single thread workload. I tried strace this, but
frankly I am not sure what to look for.

All in all I just want to bring those issues to your attention.
Whether it is fixable or not is another story. But we will not know
unless someone with required knowledge analyze it.

-Kacper

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