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SubjectRe: cleancache can lead to serious performance degradation
Thank you everybody for reviewing my report.

On 08/25/11 18:56, Dan Magenheimer wrote:
> Are your measurements on a real workload or a benchmark? Can
> you describe your configuration more (e.g. number of spindles
> -- or SSDs?). Is any swapping occurring?

I've noticed performance degradation during the real workload. I did not
run any special benchmarks to prove this problem. All my conclusions are
based on everyday usage case scenarios.

I use multi-purpose (read: all-purpose) server with Intel Core 2 Duo
E6550, 8GB DDR2, 4 1Gbps NICs and 16 1.5TB 5.4k rpm hard drives in LAN
with ~50 workstations and WAN with couple of hundreds clients.
Usually, 50 to 60% of RAM is used by "applications". Most of the rest is
used for cache. Swap allocation is usually less then 100MB (divided to
three spindles). Swapping is rare (I monitor both swap usage in MB and
swap reads/writes along with many other parameters).
Drives are partitioned and some of partitions are stand-alone, some are
in software RAID1 and some are in software RAID5, all depending of the
partition purpose. Spindles are slow, but RAID5 gives us possibility to
reach throughputs high enough to get affected with cleancache/zcache.
Just an insight in _synthetic_ RAID5 performance during the light/night
server load (not an isolated test with all other services shot down):
/dev/md2:
Timing buffered disk reads: 1044 MB in 3.00 seconds = 347.73 MB/sec
/dev/md3:
Timing buffered disk reads: 1078 MB in 3.02 seconds = 356.94 MB/sec
/dev/md4:
Timing buffered disk reads: 1170 MB in 3.00 seconds = 389.86 MB/sec

Scenarios affected by cleancache/zcache usage include:
- hashing of directconnect (DC++) shares on RAID5 arrays full of big
files like ISO images (~120MB/s without cleancache/zcache as microdc2
uses just one thread to hash)
- copying big files to my workstation using gigabit LAN with destination
to a SSD (without cleancache/zcache up to ~105MB/s via NFS and ~117MB/s
via FTP)
- copying big files between RAID5 arrays that do not have any common spindle
(without cleancache/zcache performance varies heavily based on current
server workload: 150 - 250MB/s)

In all these scenarios, using cleancache/zcache caps throughput to
60-70MB/s.

> First, I don't recommend that zcache+cleancache be used without
> frontswap, because the additional memory pressure from saving
> compressed clean pagecache pages may sometimes result in swapping
> (and frontswap will absorb much of the performance loss). I know
> frontswap isn't officially merged yet, but that's an artifact of
> the process for submitting a complex patchset (transcendent memory)
> that crosses multiple subsystems. (See https://lwn.net/Articles/454795/
> if you're not familiar with the whole transcendent memory picture.)

I'll do my best to get familiar with the whole transcendent memory story
and give it a try to frontswap as soon as I can. Unfortunately, I'm
afraid that I'll have to postpone that at least for couple of weeks.

>>> - if there's no available CPU time, just store (or throw away) to
>>> avoid IO waits;
>
> Any ideas on how to code this test (for "no available CPU time")?

I cannot help with this question as I have no practical experience in
code development, especially OS related, but maybe some approach similar
to other kernel systems could by used. For an example, cpufreq makes
decisions for CPU clock changes based on current CPU usage by sampling
recent load/usage statistics. Obviously, I have no idea if something
similar could be used with zcache, but this was my best shot. :)

> If you have any more comments or questions, please cc me directly
> as I'm not able to keep up with all the lkml traffic, especially
> when traveling... when you posted this I was at Linuxcon 2011
> talking about transcendent memory! See:
> http://events.linuxfoundation.org/events/linuxcon/magenheimer

Please, CC me directly on any further messages regarding this problem, too.

Last but not least, thank you for developing such a great feature for
Linux kernel!

Best Regards,
Nebojsa Trpkovic




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