Date: Fri, 26 Sep 2008 11:18:06 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Jeremy Chadwick <koitsu@freebsd.org> Cc: freebsd-hackers@freebsd.org, freebsd-stable@freebsd.org Subject: Re: bad NFS/UDP performance Message-ID: <200809261818.m8QII6jF004333@apollo.backplane.com> References: <E1Kj7NA-000FXz-3F@cs1.cs.huji.ac.il> <20080926081806.GA19055@icarus.home.lan> <E1Kj9bR-000H7t-0g@cs1.cs.huji.ac.il> <20080926095230.GA20789@icarus.home.lan> <E1KjDTZ-000Jqb-Bh@cs1.cs.huji.ac.il> <20080926143153.GA26978@icarus.home.lan>
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:> -vfs.nfs.realign_test: 22141777
:> +vfs.nfs.realign_test: 498351
:>
:> -vfs.nfsrv.realign_test: 5005908
:> +vfs.nfsrv.realign_test: 0
:>
:> +vfs.nfsrv.commit_miss: 0
:> +vfs.nfsrv.commit_blks: 0
:>
:> changing them did nothing - or at least with respect to nfs throughput :-)
:
:I'm not sure what any of these do, as NFS is a bit out of my league.
::-) I'll be following this thread though!
:
:--
:| Jeremy Chadwick jdc at parodius.com |
A non-zero nfs_realign_count is bad, it means NFS had to copy the
mbuf chain to fix the alignment. nfs_realign_test is just the
number of times it checked. So nfs_realign_test is irrelevant.
it's nfs_realign_count that matters.
Several things can cause NFS payloads to be improperly aligned.
Anything from older network drivers which can't start DMA on a
2-byte boundary, resulting in the 14-byte encapsulation header
causing improper alignment of the IP header & payload, to rpc
embedded in NFS TCP streams winding up being misaligned.
Modern network hardware either support 2-byte-aligned DMA, allowing
the encapsulation to be 2-byte aligned so the payload winds up being
4-byte aligned, or support DMA chaining allowing the payload to be
placed in its own mbuf, or pad, etc.
--
One thing I would check is to be sure a couple of nfsiod's are running
on the client when doing your tests. If none are running the RPCs wind
up being more synchronous and less pipelined. Another thing I would
check is IP fragment reassembly statistics (for UDP) - there should be
none for TCP connections no matter what the NFS I/O size selected.
(It does seem more likely to be scheduler-related, though).
-Matt
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