netdev - Re: [PATCH 00/16] Remove the ipv4 routing cache

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Date:	Wed, 25 Jul 2012 16:17:32 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	alexander.duyck@...il.com
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH 00/16] Remove the ipv4 routing cache

From: Alexander Duyck <alexander.duyck@...il.com>
Date: Wed, 25 Jul 2012 16:02:45 -0700

> Since your patches are in I have started to re-run my tests. I am
> seeing a significant drop in throughput with 8 flows which I expected,
> however it looks like one of the biggest issues I am seeing is that
> the dst_hold and dst_release calls seem to be causing some serious
> cache thrash.  I was at 12.5Mpps w/ 8 flows before the patches, after
> your patches it drops to 8.3Mpps.

Yes, this is something we knew would start happening.

One idea is to make cached dsts be per-cpu in the nexthops.

> I am also seeing routing fail periodically.

Every 30 seconds by chance? :-)

> I will be moving at rates listed above and suddenly drop to single
> digits packets per second.  When this occurs the trace completely
> changes and __write_lock_failed jumps to over 90% of the CPU cycles.

It's probably happening when the nexthop ARP entry expires.
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