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From Stefan Fuhrmann <stefan.fuhrm...@wandisco.com>
Subject Re: svn commit: r1694533 - /subversion/trunk/subversion/libsvn_ra_svn/marshal.c
Date Tue, 25 Aug 2015 21:08:05 GMT
./subversion/svnserve/svnserve -Tdr
/media/stefan/033c7ee9-9980-45a8-b9c6-f911dc2ac664/f7-test/ -M 1000 -c 0
--client-speed 100000
stefan@macbook:~/develop/trunk$ time ./subversion/svn/svn diff --summarize
svn://localhost/ruby-f6-packed -r0:HEAD > /dev/null

real    0m2.150s
user    0m1.964s
sys    0m0.104s
stefan@macbook:~/develop/trunk$ time ./subversion/svn/svn diff --summarize
svn://localhost/ruby-f6-packed -r0:HEAD > /dev/null

real    0m1.960s
user    0m1.764s
sys    0m0.120s

==24293== I   refs:      14,916,573,539

real    0m2.335s
user    0m2.176s
sys    0m0.100s
stefan@macbook:~/develop/trunk$ time ./subversion/svn/svn diff --summarize
svn://localhost/ruby-f6-packed -r0:HEAD > /dev/null

real    0m2.132s
user    0m1.916s
sys    0m0.128s

==5689== I   refs:      16,225,780,792

3.1Gb

On Tue, Aug 25, 2015 at 3:23 PM, Branko ─îibej <brane@wandisco.com> wrote:

> On 25.08.2015 14:10, Branko ─îibej wrote:
> > On 06.08.2015 18:10, stefan2@apache.org wrote:
> >> Author: stefan2
> >> Date: Thu Aug  6 16:10:39 2015
> >> New Revision: 1694533
> >>
> >> URL: http://svn.apache.org/r1694533
> >> Log:
> >> Fix an alignment problem on machines with 32 bit pointers but atomic 64
> >> data access that may not be misaligned.
> >>
> >> * subversion/libsvn_ra_svn/marshal.c
> >>   (read_item): Ensure that the array contents are always have the APR
> >>                default alignment.
> >>
> >> Found by: Rainer Jung <rainer.jung{_AT_}kippdata.de>
> >>
> >> Modified:
> >>     subversion/trunk/subversion/libsvn_ra_svn/marshal.c
> >>
> >> Modified: subversion/trunk/subversion/libsvn_ra_svn/marshal.c
> >> URL:
> http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_ra_svn/marshal.c?rev=1694533&r1=1694532&r2=1694533&view=diff
> >>
> ==============================================================================
> >> --- subversion/trunk/subversion/libsvn_ra_svn/marshal.c (original)
> >> +++ subversion/trunk/subversion/libsvn_ra_svn/marshal.c Thu Aug  6
> 16:10:39 2015
> >> @@ -1190,14 +1190,20 @@ static svn_error_t *read_item(svn_ra_svn
> >>      }
> >>    else if (c == '(')
> >>      {
> >> +      /* On machines with 32 bit pointers, array headers are only 20
> bytes
> >> +       * which is not enough for our standard 64 bit alignment.
> >> +       * So, determine a suitable block size for the APR array header
> that
> >> +       * keeps proper alignment for following structs. */
> >> +      const apr_size_t header_size
> >> +        = APR_ALIGN_DEFAULT(sizeof(apr_array_header_t));
> >> +
> >>        /* Allocate an APR array with room for (initially) 4 items.
> >>         * We do this manually because lists are the most frequent
> protocol
> >>         * element, often used to frame a single, optional value.  We
> save
> >>         * about 20% of total protocol handling time. */
> >> -      char *buffer = apr_palloc(pool, sizeof(apr_array_header_t)
> >> -                                      + 4 * sizeof(svn_ra_svn_item_t));
> >> -      svn_ra_svn_item_t *data
> >> -        = (svn_ra_svn_item_t *)(buffer + sizeof(apr_array_header_t));
> >> +      char *buffer = apr_palloc(pool,
> >> +                                header_size + 4 *
> sizeof(svn_ra_svn_item_t));
> >> +      svn_ra_svn_item_t *data = (svn_ra_svn_item_t *)(buffer +
> header_size);
> >>
> >>        item->kind = SVN_RA_SVN_LIST;
> >>        item->u.list = (apr_array_header_t *)buffer;
> >
> > How exactly is all this different from:
> >
> >         apr_array_make(pool, 4, sizeof(svn_ra_svn_item_t)?
> >
> > Other than relying on magic knowledge of APR implementation details?
>
> All right, so I figured that the difference is that apr_array_make does
> two allocations compared to one in this code. Still: relying on
> knowledge of APR implementation details is a really bad idea.


The structure definition of apr_array_header_t is part of
the public API, i.e. will never change.


> As it
> stands, APR will correctly resize this array if necessary; but there's
> no guarantee that, sometime in the future, resizing such arrays would
> fail horribly.
>

Even if that was true, the resize is done outside APR as
well - a few lines further down.


> I very strongly suggest that we put a plain ol' apr_array_make here and,
> if this is really perceived as such a huge *overall* performance
> problem, put the allocation hack into APR itself.
>
> I suspect that apr_palloc is fast enough that we really don't need this
> hack.
>

Is it absolutely necessary to use that hand-crafted section
of code? Of course not. Does it have a significant impact
on ra_svn throughput? Yes. The whole section of local
array management saved 20% runtime in "command-heavy"
reports like this one:

$time svn diff --summarize svn://localhost/ruby -r0:HEAD > /dev/null
real    0m1.960s
user    0m1.764s
sys    0m0.120s

Going back to apr_array_make alone sets us back by ~10%:

real    0m2.132s
user    0m1.916s
sys    0m0.128s

The reason is that the protocol uses lists for every optional
element of any struct being transmitted. Of the 36M protocol
items passing through read_item(), 16M are lists; the rest
is numbers and strings.

Based on the outrage that a 3% and 5% slowdowns caused
last year, I would expect nothing less than a cheering crowd
for a 20% speedup ;) But seriously, I'd love to keep that section
of code as it uses APR public API only and is fairly decoupled
from the rest of the code.

Alternatively, we could and maybe should replace the array
in svn_ra_svn_item_t with plain C array but that requires
us rev'ing a lot of deprecated public API in svn_ra_svn.h .

-- Stefan^2.

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