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Date:      Thu, 19 Feb 2009 17:39:46 -0500
From:      John Baldwin <jhb@freebsd.org>
To:        src-committers@freebsd.org
Cc:        svn-src-head@freebsd.org, svn-src-all@freebsd.org
Subject:   Re: svn commit: r188833 - in head/sys: kern nfsclient sys
Message-ID:  <200902191739.46808.jhb@freebsd.org>
In-Reply-To: <200902192228.n1JMSn2Q009472@svn.freebsd.org>
References:  <200902192228.n1JMSn2Q009472@svn.freebsd.org>

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On Thursday 19 February 2009 5:28:49 pm John Baldwin wrote:
> Author: jhb
> Date: Thu Feb 19 22:28:48 2009
> New Revision: 188833
> URL: http://svn.freebsd.org/changeset/base/188833
> 
> Log:
>   Enable caching of negative pathname lookups in the NFS client.  To avoid
>   stale entries, we save a copy of the directory's modification time when
>   the first negative cache entry was added in the directory's NFS node.
>   When a negative cache entry is hit during a pathname lookup, the parent
>   directory's modification time is checked.  If it has changed, all of the
>   negative cache entries for that parent are purged and the lookup falls
>   back to using the RPC.  This required adding a new cache_purge_negative()
>   method to the name cache to purge only negative cache entries for a given
>   directory.
>   
>   Submitted by:	mohans, Rick Macklem, Ricardo Labiaga @ NetApp
>   Reviewed by:	mohans

Together with the previous two changes I measured a 30% drop in the number of
RPCs for a kernel build (no modules):

x nfs.old
+ nfs.new
+------------------------------------------------------------------------------+
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          xx|
| +                                                                          xx|
| +                                                                          xx|
|++                                                                          xx|
|++                                                                          xx|
|+++                                                                         xx|
|+++                                                                         xx|
|+++                                                                         xx|
|+++                                                                        xxx|
| A                                                                          A||
+------------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x  30        250515        252053        251393     251352.13     362.86492
+  30        173990        175930        175117     175011.87     474.44092
Difference at 95.0% confidence
        -76340.3 +/- 218.321
        -30.3718% +/- 0.0868584%
        (Student's t, pooled s = 422.354)

The majority of the drop was in the number of LOOKUP RPCs performed:

x nfs.old
+ nfs.new
+------------------------------------------------------------------------------+
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          x |
| +                                                                          xx|
| +                                                                          xx|
| +                                                                          xx|
| +                                                                          xx|
|+++                                                                         xx|
|+++                                                                         xx|
|+++                                                                         xx|
|+++                                                                         xx|
|+++                                                                        xxx|
| A|                                                                         A||
+------------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x  30        116997        118613        117898     117887.47     372.34733
+  30         40791         42782         41975       41846.5     481.79805
Difference at 95.0% confidence
        -76041 +/- 222.565
        -64.503% +/- 0.188794%
        (Student's t, pooled s = 430.565)

The old client performed 0 GETATTR RPC calls during the test.  The new client
performs a mix of GETATTR and ACCESS calls.  One of the worries was that the
client might now perform more RPC calls due to not priming the access cache,
but this is not true in this test.  This next histogram is for combined totals
of ACCESS and GETATTR calls:

x nfs.attr.old
+ nfs.attr.new
+------------------------------------------------------------------------------+
|      + +                                                         x  x        |
|    + + + + +                                                     x xxx  xx   |
|    +++++++++ +                                                 x x xxx  xx   |
|+  ++++++++++ +                                          +     xxxx xxxxxxxxxx|
||_______M_A________|                                              |__MA__|    |
+------------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x  30         90823         90898         90858     90859.633     19.496212
+  30         90494         90792         90537     90543.467     50.121737
Difference at 95.0% confidence
        -316.167 +/- 19.6573
        -0.347973% +/- 0.0216348%
        (Student's t, pooled s = 38.0282)

With the new client, of the ~90k ACCESS + GETATTR calls are now split into
approximately 94% GETATTR and 6% ACCESS versus 0% GETATTR and 100% ACCESS.

All of these changes are more the product of other folks who deserve the credit
moreso than myself.

-- 
John Baldwin



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