Re: [RFC][PATCH 0/6] more detailed per-process transparenthugepage statistics

[Dave Hansen]

On Tue, 2011-02-01 at 16:38 +0100, Andrea Arcangeli wrote:
> On Mon, Jan 31, 2011 at 04:33:57PM -0800, Dave Hansen wrote:
> > I'm working on some more reports that transparent huge pages and
> > KSM do not play nicely together.  Basically, whenever THP's are
> > present along with KSM, there is a lot of attrition over time,
> > and we do not see much overall progress keeping THP's around:
> > 
> > 	http://sr71.net/~dave/ibm/038_System_Anonymous_Pages.png
> > 
> > (That's Karl Rister's graph, thanks Karl!)
> 
> Well if the pages_sharing/pages_shared count goes up, this is a
> feature not a bug.... You need to print that too in the chart to show
> this is not ok

Here are the KSM sharing bits for the same run:

	http://sr71.net/~dave/ibm/009_KSM_Pages.png

It bounces around a little bit on the ends, but it's fairly static
during the test, even when there's a good downward slope on the THP's.

Hot of the presses, Karl also managed to do a run last night with the
khugepaged scanning rates turned all the way up:

	http://sr71.net/~dave/ibm/038_System_Anonymous_Pages-scan-always.png

The THP's there are a lot more stable.  I'd read that as saying that the
scanning probably just isn't keeping up with whatever is breaking the
pages up.

> KSM will slowdown performance also during copy-on-writes when
> pages_sharing goes up, not only because of creating non-linearity
> inside 2m chunks (which makes mandatory to use ptes and not hugepmd,
> it's not an inefficiency of some sort that can be optimized away
> unfortunately). We sure could change KSM to merge 2M pages instead of
> 4k pages, but then the memory-density would decrease of several order
> of magnitudes making the KSM scan almost useless (ok, with guest
> heavily using THP that may change, but all pagecache is still 4k... so
> for now it'd be next to useless).

Yup, unless we do something special, the odds of sharing those 2MB
suckers are near zero.

> I would prefer to close the issues that you just previously reported,
> sometime with mmap_sem and issues like that, before adding more
> features though but I don't want to defer things either so it's up to
> you.

I'm happy to hold on to them for another release.  I'm actually going to
go look at the freezes I saw now that I have these out in the wild.
I'll probably stick them in a git tree and keep them up to date.

Are there any other THP issues you're chasing at the moment?

-- Dave

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