bad NFS/UDP performance

Danny Braniss danny at
Sat Sep 27 07:20:40 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 |
>     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.
it's zero, so I guess I'm ok there.
funny though, on my 'good' machine, vfs.nfsrv.realign_test: 5862999
and on the slow one, it's 0 - but then again the good one has been up
for several days.

>     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.
ahh, nfsiod, it seems that it's now dynamicaly started! at least none show
when host is idle, after i run my tests  there are 20! with ppid 0
need to refresh my NFS knowledge.
how can I see the IP fragment reassembly statistics?

>     (It does seem more likely to be scheduler-related, though).

tend to agree, I tried bith ULE/BSD, but the badness is there.

> 						-Matt


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