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Re: Fresh checkout vs 'svn upgrade': How good is good enough?

From: Paul Burba <ptburba_at_gmail.com>
Date: Tue, 14 Jun 2011 18:47:18 -0400

On Tue, Jun 14, 2011 at 11:45 AM, Paul Burba <ptburba_at_gmail.com> wrote:
> On Tue, Jun 14, 2011 at 3:41 AM, Philip Martin
> <philip.martin_at_wandisco.com> wrote:
>> Paul Burba <ptburba_at_gmail.com> writes:
>>> No surprise that your checkouts are faster than mine given you are
>>> using a local mirror.  What's puzzling me is why my upgrades are so
>>> much slower than yours.
>>> Running an upgrade of a trunk WC on my machine under xperf takes
>>> 00:03:46.351 elapsed and 11.44s CPU time using my primary drive (320
>>> GB, SATA-II, 7200 rpm, 16 MB Cache, NTFS).  Subversion spends 50s
>>> total disk service time (46.8s of that is read service time).
>> Does "total disk service time" represent all the time waiting for disk
>> IO?  11s CPU plus 50s disk still leaves 165s unaccounted.
> I've only just started using xperf (it's part of the Windows
> Performance Toolkit) and can't find an exact definition of their
> terminology.  I can say that xperf adds lot of overhead since upgrades
> run under it reliably take about twice as long.
> The 165s of unaccounted time is largely made up of 143s of disk
> service time by the system process...but now that I dig a little
> further I find that the system process' disk utilization is touching
> every directory in the working copy as well as writing the new
> .svn\pristine\ stuffs.  Previously I was only looking at the disk
> utilization by the svn.exe process itself.
> ~~~~~
> Not sure what to make of this: I tried an upgrade of a trunk WC using
> trunk_at_1135642 on my HDD and then my SSD (on the same box)
> Elapsed time for the HDD: 00:01:12.450
> For the SSD: 00:00:09.267
> Not sure if this points to anything other than SSDs are faster (duh!),
> but the degree of improvement was a unexpected.
>> Upgrade
>> prints a notification line for each directory, is there a significant
>> delay before the first line, or after the last line, or is the delay
>> spread among the lines?
> In all my testing I used the --quiet option.  I just now tried it
> without and the delay is definitely spread among the output.
>> If it is a Subversion problem rather than your machine there are two
>> areas that may be worth investigating (but it's hard to say when most of
>> the time is unaccounted).
>> The property migration currently invokes multiple transactions
>> per-directory.  Moving to a single transaction per-directory will
>> probably help.
>> Upgrade currently copies all the text-bases, I did experiment with
>> creating workqueue items to move them instead but it wasn't any faster
>> on my Linux box.  However it may help on Windows.
> A bit of cut-and-paste from IRC this morning for the benefit of others
> reading this thread:
> <philipm> Bert, pburba: I suppose we could do the whole upgrade as a
> single transaction.
> <philipm> The database will never be accessed by more than one
> thread/process during upgrade.
> <philipm> And "partial" upgrades are already thrown away.
> <Bert> philipm: Agree
> <hwright> philipm: we have nested txns in sqlite, so you should be
> able to just wrap the entire upgrade process in a txn
> <Bert> philipm: Last week I tried to reduce the number of stats in the
> property load code by just fetching all the entries in the property
> dirs, but the perf difference was not measurable. I think making it a
> single transaction would have the most impact.
> <pburba> philipm, Bert, hwright : Have we done anything similar to
> that already (wrapping the whole upgrade in a single transaction)? I'd
> like to tackle it, but looking at similar work to provide some
> understanding/traction would be very helpful
> <hwright> pburba: we don't expose txns outside of wc_db
> <philipm> pburba: What you want is upgrade_working_copy as a callback
> from svn_wc__db_with_txn
> <hwright> I'm not sure how much of the work is done in wc_db (and how
> much without), but that's the first thing to consider
> <philipm> We would have to move lots of code, or expose the txn to
> upgrade.c in some form.
> <hwright> upgrade is such a special case, it makes sense to
> potentially expose it in a limited fashion
> <Bert> svn_sqlite__with_lock()
> <philipm> upgrade.c already access the sqlite db directly
> <hwright> philipm: in that case, just do the txn in upgrade.c, and
> wc_db.c should be fine due to txn nesting, yes?
> <philipm> Yes.  Putting svn_sqlite__with_lock around
> upgrade_working_copy in upgrade.c will probably be OK
> <hwright> (if it isn't, you'll know pretty quick. :) )
> <philipm> Upgrade uses a txn to write entries, per-dir.
> <philipm> But if the whole upgrade is a txn that could be removed,
> it's not used for anything other than upgrade.
> <philipm> With nested txns that will not be necessary, but avoiding
> nested txns may have a performance benefit?
> <philipm> I don't know.  For a first attempt just put
> upgrade_working_copy in svn_sqlite__with_lock.
> <pburba> philipm: Ok, I'll give that a try then.

Here's a stab at it. In my ad-hoc testing, the attached patch
improved the elapsed upgrade time of a ^/subversion/trunk WC from
00:02:00 to 00:01:34 (3 runs). It improved an upgrade of a
^/subversion/tags/ebcdic WC from 00:11:44 to 00:06:52 (single run).

Improve 'svn upgrade' performance by moving more of the upgrade process into
a single sqlite transaction.

* subversion/libsvn_wc/entries.c

   entries_write_new_cb): Remove.

  (svn_wc__write_upgraded_entries): Don't run this part of the upgrade
   operation in a dedicated sqlite transaction, but rather embed it in a
   larger transaction. Also use the existing iterpool when creating
   tmp_entry_abspath argument to write_entry.

* subversion/libsvn_wc/upgrade.c

  (upgrade_to_wcng): Don't create the new DB here, move that to svn_wc_upgrade.

   upgrade_working_copy_txn): New baton and transaction wrapper for

  (svn_wc_upgrade): Create the new wcng DB here and wrap the call to
   upgrade_working_copy() in a sqlite transaction.


Received on 2011-06-15 00:47:49 CEST

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