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Re: 1.7.x - merge now accesses all files in WC?

From: Hyrum K. Wright <hyrum_wright_at_mail.utexas.edu>
Date: Mon, 6 Dec 2010 20:39:15 -0600

On Mon, Dec 6, 2010 at 4:28 PM, Hyrum K. Wright
<hyrum_wright_at_mail.utexas.edu> wrote:
> On Mon, Dec 6, 2010 at 3:15 PM, Daniel Becroft <djcbecroft_at_gmail.com> wrote:
>> On Mon, Dec 6, 2010 at 8:50 PM, Daniel Becroft <djcbecroft_at_gmail.com> wrote:
>>
>>> On Mon, Dec 6, 2010 at 7:13 PM, Daniel Shahaf <d.s_at_daniel.shahaf.name>wrote:
>>>
>>>> Instead of guessing which function causes the lstat() calls, could we
>>>> have a tool tell?
>>>>
>>>> I've looked at 'ltrace -S', but it seems to me that will only tell us
>>>> which public svn_wc_* API is responsible for the calls.  Perhaps there
>>>> is another tool that can help us follow the entire call chain down to
>>>> the lstat() call?
>>>>
>>>> Daniel
>>>> . o O (gdb with a breakpoint in lstat() and some fu to extract
>>>> statistics about the stack trace?)
>>>>
>>>
>>> Very true.
>>>
>>> Okay, after a crash course in using GDB, I've run some further testing.
>>> Running with breakpoints set on the lstat() system call, it looks to be the
>>> known issue that was originally pointed to. A partial stack trace is below:
>>>
>>> #0  0x00007ffff644ab45 in __lxstat (vers=<value optimised out>,
>>>     name=<value optimised out>, buf=0x7fffffffcbf0)
>>>     at ../sysdeps/unix/sysv/linux/wordsize-64/lxstat.c:38
>>> #1  0x00007ffff69269b6 in lstat (finfo=0x7fffffffccf0,
>>>     fname=0x6bd638 "/home/djcbecroft/dev/workingcopy/A/beta.txt",
>>>     wanted=33137, pool=0x6bd588) at /usr/include/sys/stat.h:464
>>> #2  apr_stat (finfo=0x7fffffffccf0,
>>>     fname=0x6bd638 "/home/djcbecroft/dev/workingcopy/A/beta.txt",
>>>     wanted=33137, pool=0x6bd588) at file_io/unix/filestat.c:292
>>> #3  0x00007ffff6fb1199 in io_check_path (
>>>     path=0x6bd608 "/home/djcbecroft/dev/workingcopy/A/beta.txt",
>>>     resolve_symlinks=0, is_special_p=0x7fffffffced8, kind=0x7fffffffcedc,
>>>     pool=0x6bd588) at subversion/libsvn_subr/io.c:222
>>> #4  0x00007ffff6fb1346 in svn_io_check_special_path (
>>>     path=0x6bd608 "/home/djcbecroft/dev/workingcopy/A/beta.txt",
>>>     kind=0x7fffffffcedc, is_special=0x7fffffffced8, pool=0x6bd588)
>>>     at subversion/libsvn_subr/io.c:283
>>> #5  0x00007ffff794513d in svn_wc__db_pdh_parse_local_abspath (
>>>     pdh=0x7fffffffd038, local_relpath=0x7fffffffd030, db=0x64ad38,
>>>     local_abspath=0x6bd608 "/home/djcbecroft/dev/workingcopy/A/beta.txt",
>>>     smode=svn_sqlite__mode_readwrite, result_pool=0x6bd588,
>>>     scratch_pool=0x6bd588) at subversion/libsvn_wc/wc_db_pdh.c:382
>>> #6  0x00007ffff7935d58 in svn_wc__db_read_info (status=0x7fffffffd184,
>>>     kind=0x7fffffffd188, revision=0x0, repos_relpath=0x0,
>>> repos_root_url=0x0,
>>>     repos_uuid=0x0, changed_rev=0x0, changed_date=0x0, changed_author=0x0,
>>>     last_mod_time=0x0, depth=0x0, checksum=0x0, translated_size=0x0,
>>>     target=0x0, changelist=0x0, original_repos_relpath=0x0,
>>>     original_root_url=0x0, original_uuid=0x0, original_revision=0x0,
>>>     props_mod=0x0, have_base=0x0, have_work=0x0, conflicted=0x0, lock=0x0,
>>>     db=0x64ad38,
>>>     local_abspath=0x6bd608 "/home/djcbecroft/dev/workingcopy/A/beta.txt",
>>>     result_pool=0x6bd588, scratch_pool=0x6bd588)
>>>     at subversion/libsvn_wc/wc_db.c:5261
>>>
>>> However, I then put the breakpont in the svn_wc__db_read_info function and,
>>> just for A/beta.txt, it gets called 7 times, when in terms leads to 7
>>> additional lstat() calls. Looking further the call stack appears as follows:
>>>
>>> #0  svn_wc__db_read_info (status=0x7fffffffd184, kind=0x7fffffffd188,
>>>     revision=0x0, repos_relpath=0x0, repos_root_url=0x0, repos_uuid=0x0,
>>>     changed_rev=0x0, changed_date=0x0, changed_author=0x0,
>>> last_mod_time=0x0,
>>>     depth=0x0, checksum=0x0, translated_size=0x0, target=0x0,
>>> changelist=0x0,
>>>     original_repos_relpath=0x0, original_root_url=0x0, original_uuid=0x0,
>>>     original_revision=0x0, props_mod=0x0, have_base=0x0, have_work=0x0,
>>>     conflicted=0x0, lock=0x0, db=0x64ad38,
>>>     local_abspath=0x6bd608 "/home/djcbecroft/dev/workingcopy/A/beta.txt",
>>>     result_pool=0x6bd588, scratch_pool=0x6bd588)
>>>     at subversion/libsvn_wc/wc_db.c:5259
>>> #1  0x00007ffff78fab96 in walker_helper (db=0x64ad38,
>>>     dir_abspath=0x6ba288 "/home/djcbecroft/dev/workingcopy/A",
>>> show_hidden=1,
>>>     walk_callback=0x7ffff7ba9026 <get_mergeinfo_walk_cb>,
>>>     walk_baton=0x7fffffffd4e0, depth=svn_depth_infinity,
>>>     cancel_func=0x4119c3 <svn_cl__check_cancel>, cancel_baton=0x0,
>>>     scratch_pool=0x6ba208) at subversion/libsvn_wc/node.c:679
>>> #2  0x00007ffff78facb5 in walker_helper (db=0x64ad38,
>>>     dir_abspath=0x67d258 "/home/djcbecroft/dev/workingcopy", show_hidden=1,
>>>
>>>     walk_callback=0x7ffff7ba9026 <get_mergeinfo_walk_cb>,
>>>     walk_baton=0x7fffffffd4e0, depth=svn_depth_infinity,
>>>     cancel_func=0x4119c3 <svn_cl__check_cancel>, cancel_baton=0x0,
>>>     scratch_pool=0x6a0dc8) at subversion/libsvn_wc/node.c:716
>>> #3  0x00007ffff78faf9a in svn_wc__internal_walk_children (db=0x64ad38,
>>>     local_abspath=0x67d258 "/home/djcbecroft/dev/workingcopy",
>>> show_hidden=1,
>>>     walk_callback=0x7ffff7ba9026 <get_mergeinfo_walk_cb>,
>>>     walk_baton=0x7fffffffd4e0, walk_depth=svn_depth_infinity,
>>>     cancel_func=0x4119c3 <svn_cl__check_cancel>, cancel_baton=0x0,
>>>     scratch_pool=0x6a0dc8) at subversion/libsvn_wc/node.c:763
>>> #4  0x00007ffff78fb06d in svn_wc__node_walk_children (wc_ctx=0x64ad20,
>>>     local_abspath=0x67d258 "/home/djcbecroft/dev/workingcopy",
>>> show_hidden=1,
>>>     walk_callback=0x7ffff7ba9026 <get_mergeinfo_walk_cb>,
>>>     walk_baton=0x7fffffffd4e0, walk_depth=svn_depth_infinity,
>>>     cancel_func=0x4119c3 <svn_cl__check_cancel>, cancel_baton=0x0,
>>>     scratch_pool=0x6a0dc8) at subversion/libsvn_wc/node.c:785
>>>
>>> Using additional breakpoints, I have verified that
>>> svn_wc__node_walk_children, svn_wc__internal_walk_children and walker_helper
>>> only get called once. It seems to be an issue within the walker_helper
>>> itself.
>>>
>>> I'll continue looking at it further - let me know if I'm barking up the
>>> wrong tree.
>>>
>>> Cheers,
>>> Daniel B.
>>> ... who now has a much greater appreciation for those that debug in GDB on
>>> a regular basis.
>>>
>>
>> I think I've found the culprit. As we are walking the children
>> (svn_wc__internal_walk_children, node.c), we call the walk_callback for each
>> node. At the moment, this is the get_mergeinfo_walk_cb function in merge.c.
>> Inside this, we call the following:
>>
>> svn_wc__node_is_status_present
>> svn_wc__node_is_status_deleted
>> svn_wc__node_is_status_absent
>> svn_wc__path_switched
>> svn_wc_read_kind
>> svn_wc__node_get_depth
>>
>> Each of these will run a separate svn_wc__db_read_info call, and therefore a
>> separate lstat(). There's also a couple of other calls that might call it
>> indirectly.
>
> So it looks an improvement would be something like
> svn_wc__get_info_for_merge_callback() which does it all in one go (or
> some other less-verbosely-named function).

Picked the low-hanging fruit in r1042892. More work could be done,
but I think the problems lie deeper, namely:
* read_info() was originally envisioned as being cheap to call.
Apparently that isn't the case anymore, part of which is no doubt
caused by...
* the call to svn_io_check_special_path() in
vn_wc__db_pdh_parse_local_abspath(). There is a comment in that file
that:

  /* ### at some point in the future, we may need to find a way to get
     ### rid of this stat() call. it is going to happen for EVERY call
     ### into wc_db which references a file. calls for directories could
     ### get an early-exit in the hash lookup just above. */

It's really unfortunate that every wc_db call on a file which fetches
a pdh has to call stat(). I tried to implement an optimization that
looks at the parent (which is much more likely to succeed, now that we
have single-db), but it failed for externals (which have their own
wc.db databases). We could work around that, but to do so, we'd need
to know if we are looking at a file or a directory...and the way to do
that is by calling stat().

Improvements, patches, suggestions encouraged.

-Hyrum
Received on 2010-12-07 03:39:58 CET

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