Re: md raid performance with 3-18-rc3

[NeilBrown <neilb@suse.de>]

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On Wed, 26 Nov 2014 13:41:39 +0530 Manish Awasthi
<manish.awasthi@caviumnetworks.com> wrote:

> Whatever data I have on comparison is attached, I have consolidated this 
> from log files to excel. See if this helps.

raid_3_18_performance.xls shows read throughput to be consistently 20% down
on 3.18 compared to 3.6.11.

Writes are a few percent better for 4G/8G files, 20% better for 16G/32G files.
unchanged above that.
Given that you have 8G of RAM, that seems like it could be some change in
caching behaviour, and not necessarily a change in RAID behaviour.

The CPU utilization roughly follows the throughput: 40% higher when write
throughput is 20% better.
Could you check if the value of /proc/sys/vm/dirty_ratio is the same for both
tests.  That number has changed occasionally and could affect these tests.


The second file, 3SSDs-perf-2-Cores-3.18-rc1 has the "change" numbers
negative where I expected positive.. i.e. negative mean an increase.

Writes consistently have higher CPU utilisation.
Reads consistently have much lower CPU utilization.

I don't know what that means ... it might not mean anything.

Could you please run the tests between the two kernels *with* RAID.  i.e.
directly on an SSD.  That will give us a baseline for what changes are caused
by other parts of the kernel (filesystem, block layer, MM, etc).  Then we can
see how much change RAID5 is contributing.

The third file, 3SSDs-perf-4Core.xls seems to show significantly reduced
throughput across the board.
CPU utilization is less (better) for writes, but worse for reads.  That is
the reverse of what the second file shows.

I might try running some tests across a set of kernel versions and see what I
can come up with.

NeilBrown

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