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Re: svn commit: r1508170 - in /subversion/trunk:

From: Branko Čibej <brane_at_wandisco.com>
Date: Thu, 01 Aug 2013 00:01:04 +0200

On 31.07.2013 23:21, Stefan Fuhrmann wrote:
> On Tue, Jul 30, 2013 at 9:56 PM, Greg Stein <gstein_at_gmail.com
> <mailto: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()
>
> Cheers,
> -g
>
> (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)
>
>
> Hi all,
>
> 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().
>
> Suggestion:
>
> * revert svn_hash_gets to simply use APR_HASH_KEY_STRING
> * where it is being used, make svn_private_config.h the first #include
>
> -- Stefan^2.
>
>
> 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

Thanks, this the kind of measurement I wanted to see.

> 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))

Interestingly enough -- depending on the compiler, strlen of a constant
string will already be optimized. So that makes me wonder ...

> 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
> 251,278 revisions
> 1,335,928 msg lines
> 0 changes
> 3 ... svn-bench null-log svn://localhost/bsd-x -v
> 251,278 revisions
> 1,335,928 msg lines
> 2,684,416 changes

... how much the differences between the three measurements is the
result of different command-line options ...

-- Brane

-- 
Branko Čibej | Director of Subversion
WANdisco // Non-Stop Data
e. brane_at_wandisco.com
Received on 2013-08-01 00:01:56 CEST

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