bad NFS/UDP performance
dillon at apollo.backplane.com
Fri Sep 26 18:30:32 UTC 2008
:> -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).
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