On 08.05.2011 18:40, Daniel Shahaf wrote:
> A few questions:
> stefan2_at_apache.org wrote on Sun, May 08, 2011 at 13:48:33 -0000:
>> Author: stefan2
>> Date: Sun May 8 13:48:33 2011
>> New Revision: 1100738
>> URL: http://svn.apache.org/viewvc?rev=1100738&view=rev
>> Make membuffer cache segmentation dynamic, i.e. don't segment
>> smaller caches. That will allow for much larger objects to be cached
>> such as large directories.
>> Remember: max cachable item size = cache size / (4 * #segments)
>> * subversion/libsvn_subr/cache-membuffer.c
>> (CACHE_SEGMENTS): drop constant
>> (MIN_SEGMENT_SIZE): introduce segmentation threshold constant
>> (svn_membuffer_t): add segment count variable
>> (get_group_index): adapt cache segment selection
>> (svn_cache__membuffer_cache_create): determine number of segments dynamically
>> (svn_membuffer_cache_get_info): adapt to variable segment count
>> Modified: subversion/trunk/subversion/libsvn_subr/cache-membuffer.c
>> URL: http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_subr/cache-membuffer.c?rev=1100738&r1=1100737&r2=1100738&view=diff
>> --- subversion/trunk/subversion/libsvn_subr/cache-membuffer.c (original)
>> +++ subversion/trunk/subversion/libsvn_subr/cache-membuffer.c Sun May 8 13:48:33 2011
>> @@ -619,7 +621,7 @@ get_group_index(svn_membuffer_t **cache,
>> memcpy(to_find, checksum->digest, APR_MD5_DIGESTSIZE);
>> /* select the cache segment to use */
>> - *cache =&(*cache)[to_find % CACHE_SEGMENTS];
>> + *cache =&(*cache)[to_find& ((*cache)->segment_count -1)];
> Why did you switch from modulo to bitwise-and ?
For speed. This is on a critical path (cache lookups are frequent
and modulo is a very expensive operation). The previous code
also used the bitwise operator because the compiler knew that
CACHE_SEGMENTS was a power of 2.
>> /* Get the group that *must* contain the entry. Fold the hash value
>> * just to be sure (it should not be necessary for perfect hashes).
>> @@ -903,16 +905,38 @@ svn_cache__membuffer_cache_create(svn_me
>> svn_boolean_t thread_safe,
>> apr_pool_t *pool)
>> - /* allocate cache as an array of segments / cache objects */
>> - svn_membuffer_t *c = apr_palloc(pool, CACHE_SEGMENTS * sizeof(*c));
>> + svn_membuffer_t *c;
>> + apr_uint32_t segment_count_shift = 0;
>> + apr_uint32_t segment_count = 1;
>> apr_uint32_t seg;
>> apr_uint32_t group_count;
>> apr_uint64_t data_size;
>> + /* Determine a reasonable number of cache segments. Segmentation is
>> + * only useful for multi-threaded / multi-core servers as it reduces
>> + * lock contention on these systems.
>> + *
>> + * But on these systems, we can assume that ample memory has been
>> + * allocated to this cache. Smaller caches should not be segmented
>> + * as this severely limites the maximum size of cachable items.
>> + *
>> + * Segments should not be smaller than 32MB and max. cachable item
>> + * size should grow as fast as segmentation.
>> + */
>> + while ((2 * MIN_SEGMENT_SIZE<< (2 * segment_count_shift))< total_size)
>> + ++segment_count_shift;
> x * y<< z, confusing to parse for precedence, but the result is the same either way.
There was a less obvious issue behind that as segment_count_shift
being 32 bits makes the whole left-hand side 32 bits only.
Fixed in r1101091.
> As to shifting by 2*segment_count_shift... if I understand correctly
> that is in order to balance the size of a segment v. the number of segments?
>  (and via that the maximum cacheable item size?)
>> @@ -942,10 +966,12 @@ svn_cache__membuffer_cache_create(svn_me
>> directory_size = group_count * sizeof(entry_group_t);
>> - for (seg = 0; seg< CACHE_SEGMENTS; ++seg)
>> + for (seg = 0; seg< segment_count; ++seg)
> Wrong indentation change.
Fixed in r1101091.
Received on 2011-05-09 18:44:22 CEST