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Re: Better choice for Linux semaphore than spinlock?

From: Doug Robinson <doug.robinson_at_wandisco.com>
Date: Mon, 7 Oct 2019 15:56:30 -0400


On Mon, Oct 7, 2019 at 2:40 PM Branko Čibej <brane_at_apache.org> wrote:

> On Mon, 7 Oct 2019, 19:47 Doug Robinson, <doug.robinson_at_wandisco.com>
> wrote:
>> I spoke with this user late last week. They stated that they can only
>> get approximately 400 parallel SVN operations before the "system
>> time" consumes all available CPU for an 8-core machine. Adding more cores
>> won't help because of the nature of spin locks (it makes things worse).
>> Turns out that even with ~100 parallel SVN operations the "system time"
>> starts becoming significant/measurable (~10%). Both HTTP (mod_dav_svn) and
>> "svnserve" protocols participate in the lock contention.
>> Your help would be greatly appreciated.
> Whew. So. Reducing this issue to "use a more efficient lock" is not going
> to work, and you provided far too little information to even attempt a
> diagnosis. For starters, I recommend gathering as much info as possible
> (anonymised of course) about the server configuration, everything from
> httpd an svnserve to the repository config and underlying filesystem, if
> possible. Getting stack traces of the "stuck" threads would be necessary,
> too. Without knowing exactly what is happening, these kinds of problems are
> extremely hard to understand, let alone fix.

I'll try to get this information and report back. Or perhaps they can join
this conversation (I gave them a pointer).

I'd be surprised if the spinlock is the actual culprit. AFAIK, kernel-level
> locks hand off to the scheduler if they spin too long; on multiprocessor
> machines, this is usually more efficient than immediately yielding and
> causing an expensive context switch. It's possible that you're seeing an
> unfortunate timing "resonance" that might go away with either more *or*
> less cores being available. The behaviour is really hard to predict.

Note: the told me that RHEL support was used and that they identified the
culprit as SVN mutex locks being translated into spin-locks at the OS level.
They also provided the example of Apache itself already having worked
around this in better ways but because this is really buried deep in
mod_dav_svn/svnserve the Apache work-arounds won't help.

Again, I'll see what I can obtain in terms of stack tracebacks, etc.



> -- Brane
>> On Fri, Oct 4, 2019 at 9:20 AM Doug Robinson <doug.robinson_at_wandisco.com>
>> wrote:
>>> Folks:
>>> From a Subversion user:
>>> “... we have very high concurrent connections to Subversion that seem to
>>> crater Subversion. The SVN Serve process we use to access the Subversion
>>> repository is using the “svn” protocol by our “system user”, mostly
>>> read-only. Then, we, on behalf of the user make request to Subversion
>>> using the “http” protocol to fetch their data. So we have lots of
>>> connections to Subversion. But the volume of concurrent requests over the
>>> “svn” protocol cause the “svnserve” process to consume CPU cycles in a
>>> kernel “mutex-lock” which is implemented using “spin locks”. The “svnserve”
>>> process makes the mutex calls using the “apache” (APR) semaphore wait
>>> calls, but on Linux this is a “mutext-lock” request.”
>>> So is there a better, more scalable, semaphore that can be used?

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Received on 2019-10-07 21:56:46 CEST

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