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From: Robert Watson <rwatson@FreeBSD.org>
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To: Anton Yuzhaninov <citrin@citrin.ru>
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Cc: freebsd-net@freebsd.org
Subject: Re: sysctl net.inet.tcp.syncache.count
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On Fri, 7 Nov 2008, Anton Yuzhaninov wrote:

> % sysctl net.inet.tcp.syncache
> net.inet.tcp.syncache.rst_on_sock_fail: 1
> net.inet.tcp.syncache.rexmtlimit: 3
> net.inet.tcp.syncache.hashsize: 1024
> net.inet.tcp.syncache.count: -84
> net.inet.tcp.syncache.cachelimit: 102400
> net.inet.tcp.syncache.bucketlimit: 100
>
> Why number of entries in syncache is negative?

The syncache entry is no longer protected by a single lock, since the locks on 
the syncache are now per-hash bucket, so it looks like it's been corrupted. 
This won't lead to other problems, as the count is updated as a statistic but 
never used to, for example, enforce limits.  We should probably eliminate the 
statistic, since UMA already maintains the syncache entry count coherently as 
part of its own statistics, along with other useful information:

robert@fledge:~> vmstat -z | head -1 ; vmstat -z | grep -i syncache
ITEM                     SIZE     LIMIT      USED      FREE  REQUESTS  FAILURES
syncache:                 104,    15392,        0,      222,  2638489,        0

I'm not convinced its worth adding the overhead of coherent statistics 
gathering for the cache entry count if we already have it elsewhere.

Robert N M Watson
Computer Laboratory
University of Cambridge