Re: [PATCH v3 3/3] NFSD: Add support for encoding multiple segments

[Dave Chinner <david@fromorbit.com>]

On Wed, Apr 15, 2015 at 03:32:02PM -0400, Anna Schumaker wrote:
> I just ran some more tests comparing the directio case across
> different filesystem types.  These tests used three 1G files:
> 100% data, 100% hole, and mixed file with alternating 4k data and
> hole segments.  The mixed case seems to be consistently slower
> compared to NFS v4.1, and I'm at a loss for anything I could do to
> make it faster.  Here are my numbers:
> 
> ###########
> #         #
> #   XFS   #
> #         #
> ###########
> 
> 
> NFS v4.1:
>                             Trial
> |---------|---------|---------|---------|---------|---------|---------|
> |         |    1    |    2    |    3    |    4    |    5    | Average |
> |---------|---------|---------|---------|---------|---------|---------|
> |    Data |  1.883s |  1.808s |  1.781s |  1.685s |  1.591s |  1.746s |
> |    Hole |  1.815s |  1.635s |  1.682s |  1.698s |  1.653s |  1.697s |
> |   Mixed |  2.089s |  2.024s |  1.970s |  1.925s |  2.049s |  2.011s |
> |---------|---------|---------|---------|---------|---------|---------|
> 
> 
> NFS v4.2:
>                             Trial
> |---------|---------|---------|---------|---------|---------|---------|
> |         |    1    |    2    |    3    |    4    |    5    | Average |
> |---------|---------|---------|---------|---------|---------|---------|
> |    Data |  1.849s |  1.879s |  1.852s |  1.799s |  1.781s |  1.832s |
> |    Hole |  0.668s |  0.600s |  0.611s |  0.619s |  0.617s |  0.623s |
> |   Mixed |  5.913s |  5.811s |  5.952s |  5.962s |  5.806s |  5.889s |
> |---------|---------|---------|---------|---------|---------|---------|

What that says to me is that the READ_PLUS when there are (worst
case) mixed holes is either burning a lot more CPU than we expected
or it is serialising somewhere (not sure where, everything in XFS
should be shared locks on read/seek).

Can you run a perf profile (even just a snapshot from perf top) on
the server so we can see a bit about what is happening on the CPU
for the different workloads?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com