Re: High usage of mbufs
From: Robert Watson (rwatson_at_FreeBSD.org)
Date: 05/29/05
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Date: Sun, 29 May 2005 16:44:14 +0100 (BST) To: "Matthew D. Fuller" <fullermd@over-yonder.net>
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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- Previous message: Scott Long: "Re: FreeBSD router performance and PCI-Express NICs"
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