Compcache makes system very slow with huge unused Vmalloc. about compcache

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> Also VMalloc is much larger than normal (even after rmmod compcache and tlsf):

> VmallocTotal:   770040 kB
> VmallocUsed:      5164 kB
> VmallocChunk:   737848 kB

This is normal. It shows that vmalloc area total size is ~770M (normal) out of 
wich
only ~5M is used. 'VmallocChunk' shows largest contiguous virtual memory area
available (if I understand correctly).


> I notice that machine becomes very slow, even if I remove compcache.
I think this is the explanation for this:

- Compcache compresses only swap cache pages and hence space for page cache
(filesystem backed) pages reduces. So, if application working set from page 
cache
pages is greater than (TotalMem - compcacheUsedMem) then system begins to 
thrash. I
have observed this behaviour on my system also -- particulary when I set 
compcache to
a big ratio caompared with TotalRAM (say >65%).

- When you disable compcache, all those pages are decompressed which is beyond 
what
RAM can handle (thats why they were compressed in first place!). Now:
   * If swap hdisk is present, they are now again swapped to now slow hdisk
system. If these pages are part some applications working set then it causes 
system
thrashing.
   * For swapless system, these additional pages will force page-cache pages to be
reclaimed/flushed and system again thrashes.

So, if you observe slow down of system for your particular workload so can try 
these:
- Temporarily increase swappiness to, say 80 (echo 80 > /proc/sys/vm/swappiness)
- Try using smaller compcache (say 40% of RAM).

If your system still thrashes ot you get oom-kills, then your memory required 
are
just too much for even compression business to handle!

Original comment by [email protected] on 6 Mar 2008 at 9:29

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comment #1 gives reason for this issue.

Original comment by [email protected] on 8 Mar 2008 at 11:42

• Changed state: WontFix