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Bug 377975

Summary: gentoo-sources-3.0 - cleancache can lead to serious performance degradation
Product: Gentoo Linux Reporter: Nebojsa Trpkovic <trxman>
Component: New packagesAssignee: Gentoo Kernel Bug Wranglers and Kernel Maintainers <kernel>
Status: RESOLVED UPSTREAM    
Severity: normal    
Priority: Normal    
Version: unspecified   
Hardware: All   
OS: Linux   
Whiteboard:
Package list:
Runtime testing required: ---

Description Nebojsa Trpkovic 2011-08-06 12:03:57 UTC
my C2D 4MB L2 cache 2.33GHz CPU cannot cope with the amount of data it needs to compress during heavy sequential IO when cleancache/zcache are enabled.

for an example, I get 60-70MB/s from my RAID arrays and both CPU cores are saturated with system (kernel) time.
without cleancache, each RAID gives me more then 300MB/s.

so, using cleancache in scenarios with a lot of non-random data throughput can lead to very ugly performance degradation.


I understand that it's on upstream to take care of it, but I wanted to report it here to be sure if this idea is good enough.

the idea is that cleancache/zcache should implement some kind of compression throttling vavle:
- if there's available CPU time, then compress;
- if there's no avalable CPU time, just store (or throw away);


at least, there should be a warning in kernel help about this kind of situations.
Comment 1 Nebojsa Trpkovic 2011-08-30 04:09:43 UTC
as I've seen great interest in this report here :), I've reported it upstream nevertheless:

https://lkml.org/lkml/2011/8/17/351

closing this bug for gentoo.