linux-kernel - KSM & hugepages

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Message-ID: <4BA8A6DA.1000408@msgid.tls.msk.ru>
Date:	Tue, 23 Mar 2010 14:32:42 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm@...r.kernel.org>
Subject: KSM & hugepages

Hello.

I noticed an interesting thing here, with qemu-kvm, KSM and
hugepages.

When I initially enabled KSM, for my two windows guests I've
seen ~100 000 pages in /sys/kernel/mm/ksm/pages_shared .
That's quite good, and overall memory usage improved.

Now, I also enabled hugepages in kvm, which speed things
up quite significantly (the speedup is noticeable).

But now, when both KSM and hugepages are activated, I don't
see KSM in action anymore.  /sys/../mm/pages_shared shows
56 pages, which is nothing.

So I wonder what's up:
 o that's 56 _huge_ pages (which means the actual saving
   is 56*2M = 112Mb, which isn't really bad).  If that's
   the case, /sys/../mm/ interface lacks proper units
   reporting;
 o due to large pages there's much less chance to find
   two pages with identical contents, so very little can
   be shared;
 o KSM does not scan hugepages at all
 o something else.

What is the issue here?

Thanks!

/mjt
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