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Re: svn commit: r1415864 - /subversion/trunk/subversion/libsvn_ra_serf/update.c

From: Justin Erenkrantz <justin_at_erenkrantz.com>
Date: Sat, 1 Dec 2012 08:31:26 -0500

On Sat, Dec 1, 2012 at 5:59 AM, Johan Corveleyn <jcorvel_at_gmail.com> wrote:

> I'm wondering whether your concerns apply to both internet-wide
> deployments and local (all on the same LAN) ones.

That line is certainly a fair one to draw in the sand. That said, I think
the internal use case cries out even *more* for the parallel updates as the
internal server in that environment is often wildly over-provisioned on the
CPU side - with a fairly low-traffic environment, you want to take
advantage of the parallel cores of a CPU to drive the updates.

Generally speaking, what I discovered years ago back in 2006 (yikes) and I
believe is still true as we near 2013 (shudder), if everything else is
perfectly optimized (disk, latency, bandwidth, etc.), you're going to
eventually bottleneck on the checksumming on both client and server - which
is entirely CPU-bound and is expensive. You can solve that by splitting
out the work across multiple cores - for a server, you need to utilize
multiple parallel requests in-flight; and for a client, you then need to
parallelize the editor drive.

The reason that disk isn't such a bottleneck as you might first expect is
due to the OS's buffer cache - for reads on the server-side, common data is
already going to be in RAM so hot spots in the fsfs repos will already be
in memory, for writes on the client-side, modern client OSes won't
necessarily block you until everything is sync'd to disk. But, once you
exhaust the capabilities of RAM, your underlying disk architecture matters
a lot and one that might not be intuitive to those that haven't spent a lot
of time closely with them. (Hi Brane!) If you are using direct-attached
storage locally on either server or client, then you will probably be
bottlenecked right there. However, if your corporate environment has an
NFS filer or SAN (a la NetApp/EMC) backing the FSFS repository or as NFS
working copies (oh so common), those large disk subsystems are geared
towards parallel I/Os - not single-threaded I/O performance -
Isilon/BlueArc-class storage is however; but I've yet to see anyone
obsessed enough about SVN I/O perf to place either their repository or
working copies on a BlueArc-class storage system! So, if you are not using
direct-attached storage and are using NFS today in a corporate environment
on either client or server, then you want to parallelize everything so that
you can take advantage of the disk/network I/O architecture preferred by
NetApp/EMC. Throwing more cores against a NetApp/EMC storage system in a
high-available bandwidth environment allows for linear performance returns
(i.e., reading/writing one I/O is 1X, two threads is 2X, three threads is
3X, etc, etc.).

To that end, I'd eventually love to see ra_serf drive the update editor
across multiple threads so that the checksum and disk I/O bottleneck can be
distributed across cores on the client-side as well. Compared to where we
were in 2006, that's the biggest inefficiency we have yet to solve and take
advantage of. And, I'm sure this'll break all sorts of promises in the Ev1
and perhaps Ev2 world and drive C-Mike even crazier. =) But, if you want
to put a rocket pack on our HTTP performance, that's exactly what we should
do. I'm reasonably certain that serf itself could be finely tuned to
handle network I/O in a single thread at or close to wire-speed even on a
10G connection with a modern processor/OS - it's what we do with the file
contents/textdeltas that needs to be shoved to a different set of worker
threads and remove all of that libsvn_wc processing from blocking network
traffic processing and get it all distributed and thread-safe. If we do
that, woah, I'd bet that we are we going to make things way faster across
the board and completely blow everything else out of the water when our
available bandwidth is high - which is the case in an internal network.
 And, yes, that clearly could all be done in time for 1.8 without
jeopardizing the timelines one tiny bit. =P

So, that's my long-winded answer of saying that, yah, even in an internal
LAN environment, you still want to parallelize.

However, I'm definitely not going to veto a patch that would add an httpd
directive that allows the server to steer the client - unless overridden by
the client's local config - to using parallel updates or not. -- justin
Received on 2012-12-01 14:32:04 CET

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