On Tue, Jul 30, 2013 at 9:56 PM, Greg Stein <gstein_at_gmail.com> wrote:
> It really doesn't make that much of a difference. APR_HASH_KEY_STRING
> simply tells the hash function to watch for NUL rather than to
> traverse N characters. Because N doesn't have to be tracked, you might
> even argue the NUL-test is faster.
> IOW, passing APR_HASH_KEY_STRING does NOT perform a strlen(). So yes:
> a lot of this work isn't saving much at all.
> Just look at apr/tables/apr_hash.c:hashfunc_default()
> (tho we'll leave it to Stefan and Daniel to decide whether to blow
> more of their time on this "issue" or just back it all out)
There are a number of different issues to be addressed here.
* svn_hash_gets / svn_hash_sets have been introduced to
simplify coding. Being used in 600+ places, this surely is
a useful API. We should have kept it private, though.
* Definitions in svn_private_config.h determine various properties
of SVN's internal code. We must ensure that those take effect
consistently. I see two options:
- #include that header immediately after the system headers.
r1508225 follows that idea.
- no headers whatsoever, including lib-internal ones, may
depend on anything inside svn_private_config.h. Ever.
* How relevant is hash performance for SVN performance?
As it turns out, using the faster hash function behind
svn_hash__make gives a 50+% overall speedup in loggy
operations. The using strlen() in the getter is much less
of an issue (~1%). See details below. Tons of tuning on the
caller side changed the usage pattern that it works well with
SVN's own hash function, including its use of strlen().
* revert svn_hash_gets to simply use APR_HASH_KEY_STRING
* where it is being used, make svn_private_config.h the first #include
Performance measurements of loggy operations on an FSX
mirror of the FreeBSD repository using svnserve
(-Td --cache-revprops yes -c 0 -M 6000) at localhost.
Optimized build on my old 2.4GHz laptop.
Best total execution times of runs 2 to 4.
A / B / C
(1) 12.359s / 7.988s / 7.940s
(2) 1.105s / 0.783s / 0.775s
(3) 7.406s / 4.397s / 4.288s
A ... apr_hash_get(ht, key, APR_HASH_KEY_STRING) and
#define svn_hash__make apr_hash_make
B ... apr_hash_get(ht, key, APR_HASH_KEY_STRING)
C ... apr_hash_get(ht, key, strlen(key))
1 ... svn-bench null-log svn://localhost/bsd-x/head -v -g -r250000:200000
176,481 revisions, 141,842 merged in 607 merges
936,522 msg lines, 708,473 in merged revisions
959,510 changes, 703,949 in merged revisions
2 ... svn-bench null-log svn://localhost/bsd-x
1,335,928 msg lines
3 ... svn-bench null-log svn://localhost/bsd-x -v
1,335,928 msg lines
Received on 2013-07-31 23:22:20 CEST