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Re: Subversion and KeepAlive

From: Branko Čibej <brane_at_wandisco.com>
Date: Wed, 03 Jul 2013 16:07:37 +0200

On 03.07.2013 15:35, Mark Phippard wrote:
> I have noted KeepAlive coming up in a few of the Serf threads.
> I have been looking at setting up a load-balanced Subversion cluster,
> somewhat like what Blair describes in this wiki:
> http://www.orcaware.com/svn/wiki/Server_performance_tuning_for_Linux_and_Unix
> Anyway, in looking at load balancers, I noted this in the
> documentation for HAProxy:
> http://haproxy.1wt.eu/
> Keep-alive was invented to reduce CPU usage on servers when CPUs were
> 100 times slower. But what is not said is that persistent connections
> consume a lot of memory while not being usable by anybody except the
> client who openned them. Today in 2009, CPUs are very cheap and memory
> is still limited to a few gigabytes by the architecture or the price.
> If a site needs keep-alive, there is a real problem. Highly loaded
> sites often disable keep-alive to support the maximum number of
> simultaneous clients. The real downside of not having keep-alive is a
> slightly increased latency to fetch objects. Browsers double the
> number of concurrent connections on non-keepalive sites to compensate
> for this. With version 1.4, keep-alive with the client was introduced.
> It resulted in lower access times to load pages composed of many
> objects, without the cost of maintaining an idle connection to the
> server. It is a good trade-off. 1.5 will bring keep-alive to the
> server, but it will probably make sense only with static servers.
> I do not really have any comments about this, just thought I would share it.

I do have a comment; it specifically assumes the clients are browsers,
where you cannot in really predict when the next "GET" will arrive.

With Subversion, Serf and HTTPv2, you can predict that, and the
"slightly increased latency" has a much larger impact on performance.
This was demonstrated any number of times during ra_serf development.

-- Brane

Branko Čibej | Director of Subversion
WANdisco // Non-Stop Data
e. brane_at_wandisco.com
Received on 2013-07-03 16:08:14 CEST

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