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From Branko ─îibej <br...@apache.org>
Subject Re: [PATCH] Proof of concept of the better-pristines (LZ4 + storing small pristines as BLOBs) (Was: Re: svn commit: r1843076)
Date Tue, 23 Oct 2018 16:12:53 GMT
On 22.10.2018 22:14, Evgeny Kotkov wrote:
> Branko ─îibej <brane@apache.org> writes:
>
>> Still missing is a mechanism for the libsvn_wc (and possibly
>> libsvn_client) to determine the capabilities of the working copy at
>> runtime (this will be needed for deciding whether to use compressed
>> pristines).
> FWIW, I tried the idea of using LZ4 to compress the pristines and storing small
> pristines as blobs in the `PRISTINE` table.  I was particularly interested in
> how such change would affect the performance and what kind of obstacles
> would have to be dealt with.

Nice! I did some simpler tests by compressing exported trees, but this
is definitely better.

> In the attachment you will find a more or less functional implementation of
> this idea that might be useful to some extent.  The patch is a proof of
> concept: it doesn't include the WC compatibility bits and most certainly
> doesn't have everything necessary in place.  But in the meanwhile, I think
> that is might give a good approximation of what can be expected from the
> approach.
>
> The patch applies to the `better-pristines` branch.
>
> A couple of observations:
>
>  - As expected, the combined size of the pristines is halved when the data
>    itself is compressible, thus making the working copy 25% smaller.

Yes, that was my observation as well. In fact, though, storing small
BLOBs in the database itself should have even better effects, since the
space on disk actually used by a file is rounded up to the nearest
cluster size, but SQLite's blocks are typically much smaller than that.


>  - A variety of the callers currently access the pristine contents by reading
>    the corresponding files.  That doesn't work in case of compressed pristines
>    or pristines stored as BLOBs.
>
>    I think that ideally we would want to use streams as much as possible, and
>    only spill the uncompressed pristine contents to temporary files when we
>    need to pass them to external tools, etc.; and that temporary files need
>    to be backed by a work queue to avoid leaving them in place in case of an
>    application crash.

Yes and yes. Keeping those temporary spilled files on disk could turn
out to be a problem, finding a reasonable time to delete them without
having to run cleanup will be rather important, I think.


>    The patch does that kind of plumbing to some extent, but that part of the
>    work is not complete.  The starting point is around wc_db_pristine.c:
>    svn_wc__db_pristine_get_path().
>
>  - Using BLOBs to store the pristine contents didn't have a measurable impact
>    on the speed of the WC operations such as checkout in my experiments on
>    Windows.  These experiments were not comprehensive, and also I didn't run
>    the tests on *nix.

I wouldn't expect much change in performance but would expect better use
of the disk, as explained above.

>  - There's also the deprecated svn_wc_get_pristine_copy_path() public API that
>    would require plumbing to maintain compatibility; the patch performs it by
>    spilling the pristine contents result into a temporary file whose lifetime
>    is attached to the `result_pool`.

Ack; that's one reasonable definition of "lifetime." But I suspect that
any users of that function expect the pristine file to survive at least
to the next WC cleanup.

>  (I probably won't be able to continue the work on this patch in the nearby
>  future; posting this in case it might be useful.)

Thanks, it definitely is useful!

-- Brane


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