On 27.06.2011 13:54, Daniel Shahaf wrote:
Stefan Fuhrmann wrote on Sun, Jun 26, 2011 at 23:00:24 +0200:
On 11.06.2011 14:53, Daniel Shahaf wrote:
Stefan Fuhrmann wrote on Sat, Jun 11, 2011 at 10:13:27 +0200:
On 10.06.2011 22:28, Daniel Shahaf wrote:
stef...@apache.org wrote on Wed, May 25, 2011 at 22:20:25 -0000:
Author: stefan2
Date: Wed May 25 22:20:25 2011
New Revision: 1127709
URL: http://svn.apache.org/viewvc?rev=1127709&view=rev
Log:
Fix a pool usage issue: svn_cache__get_partial() may be called many
times in a row. Thus, the internal pool used to construct keys should
be cleared in this function as well from time to time.
* subversion/libsvn_subr/cache-membuffer.c
(svn_membuffer_cache_get_partial): regularly clear the internal scratch pool
Modified:
subversion/trunk/subversion/libsvn_subr/cache-membuffer.c
Modified: subversion/trunk/subversion/libsvn_subr/cache-membuffer.c
URL:
http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_subr/cache-membuffer.c?rev=1127709&r1=1127708&r2=1127709&view=diff
==============================================================================
--- subversion/trunk/subversion/libsvn_subr/cache-membuffer.c (original)
+++ subversion/trunk/subversion/libsvn_subr/cache-membuffer.c Wed May 25
22:20:25 2011
@@ -1668,6 +1668,12 @@ svn_membuffer_cache_get_partial(void **v
DEBUG_CACHE_MEMBUFFER_INIT_TAG
+ if (++cache->alloc_counter> ALLOCATIONS_PER_POOL_CLEAR)
+ {
+ apr_pool_clear(cache->pool);
+ cache->alloc_counter = 0;
+ }
+
Does this need to be guarded by a cache lock?
No. This happens in the outer / front-end code
that merely adds a key prefix (combine_key below)
before calling the shared cache object.
All front-end operations assume single-threaded
access, which should be o.k. for fs_t-local objects.
Okay, if that function is guaranteed (perhaps by API contract) not to
run concurrently to any other `front end' function (and in particular to
itself), then my concerns are resolved.
(I was worried about accessing cache->pool and cache->alloc_counter from
multiple threads concurrently --- writer-writer or writer-reader ---
which might to undefined behaviour.)
What guarantees the single-threaded access? I don't see it documented
in svn_cache.h (on the contrary, that one has explicit 'thread_safe'
parameters) and the code doesn't take a lock at at that point either.
I implemented optional thread-safety some days ago but it
turned out to require / suggest a new API for svn_mutex_t
structures (handles NULL, APR w/o threading etc.)
Otherwise, there would be even more duplicated mutex
handling code.
However, to keep API churn low for 1.7 stabilization, I like
to suggest the following: simply adding a thread_safe
parameter to the membuffer cache constructor API and
returning "not supported / implemented" if set to TRUE.
In other words, you're suggesting to have *all* membuffer caches declare
themselves as non-thread-safe in 1.7?
Since this is a completely private API, I decided to not
change the definition for 1.7. Currently, all these "frontend"
objects (forwarding to the already thread-safe singleton)
are used from a single thread.
We just apply the same guarantees that we already gave
to the inprocess_cache in 1.6. The latter provides optional
synchronization but that is never activated.
Assuming the cache users are fine with that, it solves the issue
I raised, and all these are private API's, so +1.
Also: creating the membuffer singleton respects the public API's config
singleton's SINGLE_THREADED parameter... so we'd have to somehow
"ignore" that parameter (and rely on FSFS-level mutexes?). Hmm.
No, the singleton already uses a thread-safe API.
This is not going to change soon.
-- Stefan^2.
