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SubjectRe: [PATCH v2] NFS: introduce writeback wait queue
Date
On Oct 5, 2009, at 3:11, "Wu Fengguang" <fengguang.wu@intel.com> wrote:

> Hi all,
>
> This version makes two standalone functions for easier reuse.
>
> Before patch, nr_writeback is near 1G on my 2GB laptop:
>
> nr_writeback nr_dirty nr_unstable
> 203994 2 154469
> 203994 2 154469
>
> After patch, nr_writeback is limited to nfs_congestion_kb=42MB.
>
> nr_writeback nr_dirty nr_unstable
> 11180 34195 11754
> 9865 36821 8234
> 10137 36695 9338
>
> One minor problem I noticed is, NFS writeback is not very smooth.
> This per 0.1s sampled trace shows that it can sometimes stuck for
> up to 0.5s:
>
> nr_writeback nr_dirty nr_unstable
> 11055 37408 9599
> 10311 37315 10529
> 10869 35920 11459
> 10869 35920 11459
> 10869 35920 11459
> 10869 35920 11459
> 10869 35920 11459
> 10838 35891 10042
> 10466 35891 10414
> 10900 34744 11437
> 10249 34744 12088
> 10249 34744 12088
> 10249 34744 12088
> 10249 34744 12088
> 10249 34744 12088
> 10249 34744 12088
> 10133 34743 10663
> 10505 34743 11035
> 10970 34991 11345
> 10691 34991 11593
> 10691 34991 11593
> 10691 34991 11593
> 10691 34991 11593
> 10691 34991 11593
>
> Trond, I guess nr_writeback/nr_unstable are decreased in async RPC
> "complete" events. It is understandable that nr_dirty can sometimes
> stuck on local waits, but the "local determined" nr_dirty and "remote
> determined" nr_writeback/nr_unstable tend to stuck at the same time?
> Did I miss something (that could be obvious to you)?

It looks (at 7am in the morning after getting up at 4:30am) as though
the number of unstable pages is remaining constant, which would mean
that we're sending a lot of COMMIT requests (see nfsstat). Since
COMMIT is essentially an fsync call, it means that the server is going
to be slower.

Cheers
Trond


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