netdev - Re: [PATCH] ipv4: Remove unnecessary code from rt_check

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Message-ID: <1340701893.10893.296.camel@edumazet-glaptop>
Date:	Tue, 26 Jun 2012 11:11:33 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] ipv4: Remove unnecessary code from rt_check_expire().

On Tue, 2012-06-26 at 01:56 -0700, David Miller wrote:

> Keep in mind that rt_check_expire() was written before we had realtime
> GC.  It used to be main component which kept hash chains of reasonable
> size before real ->gc() triggers.
> 
> I have about 15 entries in my routing cache, there is no reason they
> should be purged.
> 
> And consider TCP before early demux, so a connection doing financial
> trades is idle for 5 minutes.  Do you really think the next trade
> should have this pointless added latency just because we can't be
> bothered to make rt_check_expired() smarter?

We did a lot of work in this area, for example making this gc running in
process context instead of timer, and removing the ip_rt_secret_interval
that was invalidating the whole cache every 10 minutes.

I think I stopped trying to improve route cache because your intention
was to get rid of it.

Now it seems we should keep it for a while, so it makes sense to add
more fuel on it ;)

About financial guys, they probably are smart enough to :

echo bigvalue >/proc/sys/net/ipv4/route/gc_timeout

(although the cache misses on old cached dst might kill their latency
anyway)

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