Re: [PATCH 0/3 v3] dcache: make it more scalable on large system
From: Waiman Long
Date: Wed May 29 2013 - 16:33:11 EST
On 05/29/2013 12:18 PM, Simo Sorce wrote:
On Wed, 2013-05-29 at 11:55 -0400, Waiman Long wrote:
My patch set consists of 2 different changes. The first one is to avoid
taking the d_lock lock when updating the reference count in the
dentries. This particular change also benefit some other workloads that
are filesystem intensive. One particular example is the short workload
in the AIM7 benchmark. One of the job type in the short workload is
"misc_rtns_1" which calls security functions like getpwnam(),
getpwuid(), getgrgid() a couple of times. These functions open the
/etc/passwd or /etc/group files, read their content and close the files.
It is the intensive open/read/close sequence from multiple threads that
is causing 80%+ contention in the d_lock on a system with large number
of cores.
To be honest a workload base on /etc/passwd or /etc/group is completely
artificial, in actual usage, if you really have such access you use
nscd or sssd with their shared memory caches to completely remove most
of the file access.
I have no beef on the rest but repeated access to Nsswitch information
is not something you need to optimize at the file system layer and
should not be brought up as a point in favor.
The misc_rtns_1 workload that I described here is just part of a larger
workload involving other activities. It represents just 1/17 of the
total jobs that were spawned. This particular job type, however,
dominates the time because of the lock contention that it created. I
agree that it is an artificial workload as most benchmarks are. It is
certainly an exaggeration of what a real workload may be, but it doesn't
mean that similar contention will not happen in the real world
especially when the trend is to have more and more CPU cores packed in
the same machine.
Regards,
Longman
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