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Re: FSFS caching and apr_thread_rwlock_t performance on Windows

From: Ivan Zhakov <ivan_at_visualsvn.com>
Date: Fri, 19 Dec 2014 19:26:14 +0300

On 19 December 2014 at 18:55, Branko Čibej <brane_at_wandisco.com> wrote:
> On 19.12.2014 15:37, Ivan Zhakov wrote:
>> Originally FSFS caching implementation was using apr_thread_mutex_t to
>> serialize access to shared data. In r1346122 [1] implementation was
>> switched to apr_thread_rw_lock_t to improve performance. This change
>> was released in Subversion 1.8.0
>> Unfortunately current apr_thread_rwlock_t implementation is very slow
>> on Windows. As quick workaround FSFS code was patched to select
>> apr_thread_mutex_t or apr_thread_rwlock_t() depending of platform on
>> compile time (r1611380 [2]).
>> In further investigation Bert found why apr_thread_rwlock_t() so slow
>> on Windows: implementation uses kernel level mutex object instead of
>> lightweight critical section (critical sections are also used in
>> apr_thread_mutex_t in most cases).
>> The simple patch to switch apr_thread_rwlock_t() implementation to use
>> critical sections (through apr_thread_mutex_t) was proposed on APR
>> development mailing list [3], but patch was not commited nor reviewed.
>> So the current situation is:
>> 1. APR has performance problem on Windows that hurts Subversion 1.8.x and trunk
>> 2. Subversion trunk has workaround for this specific problem at compile time
>> 3. Patch proposed to APR mailing list, without any reaction for three months
>> From my point of view the proposed patch is straightforward and Bert
>> stated that it makes apr_thread_rwlock_t 10 to 140 times more
>> efficient on Windows. So the best option will be to commit Bert's
>> patch, but I cannot do this since I'm not APR committer :(
>> Thoughts?
> Thanks for the reminder ... I remember that thread on apr-dev, but it
> slipped my mind. If I don't review and commit Bert's patch in the next
> week or so, feel free to send another nagging e-mail. :)
Will do!

> FWIW, committing to APR trunk isn't enough: should be back-ported to 1.6
> and 1.5, too.
Yes, you are right. We need this to be backported to 1.6 and 1.5 APR branches.

Ivan Zhakov
Received on 2014-12-19 17:27:05 CET

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