Date: Sun, 29 May 2005 16:44:14 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: "Matthew D. Fuller" <fullermd@over-yonder.net> Cc: Michael VInce <mv@roq.com>, freebsd-performance@freebsd.org Subject: Re: High usage of mbufs Message-ID: <20050529164207.A52379@fledge.watson.org> In-Reply-To: <20050528221027.GB47448@over-yonder.net> References: <4297E2C4.1030505@roq.com> <20050528221027.GB47448@over-yonder.net>
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On Sat, 28 May 2005, Matthew D. Fuller wrote: > On Sat, May 28, 2005 at 01:17:24PM +1000 I heard the voice of > Michael VInce, and lo! it spake thus: >> On one of my web servers I have a really high usage of mbuf clusters in >> use on a web server that does about 3million hits a day. >> 4294914731/262144 mbuf clusters in use (current/max) > > No it doesn't (have really high usage, that is). It just has messed up > statistics, which is standard on SMP machines; see the archives. I have two sets of patches that correct this problem: - One uses atomic operations to synchronize access to the mbuf-related counters instead of performing non-atomic updates, which results in races. - One uses per-cpu statistics in the mbuf allocator, and critical sections to synchronize updates. Both encounter a several percent performance hit in packet-intensive workloads, so I've been reluctant to commit either. However, most of the information required for mbuf statistics is already gathered by UMA, which is now used as the back-end for the mbuf allocator. As such, the better answer is to use the UMA statistics and then see if we still need the mbuf layer stats. I haven't had a chance to investigate this in detail yet, but I'd like to get something in before 6.0. Robert N M Watson
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