On Wed, 20 Aug 2008 20:05:51 +0900
KOSAKI Motohiro [off-list ref] wrote:

Hi Cristoph,

Thank you for explain your quicklist plan at OLS.

So, I made summary to issue of quicklist.
if you have a bit time, Could you please read this mail and patches?
And, if possible, Could you please tell me your feeling?


--------------------------------------------------------------------

Now, Quicklist store some page in each CPU as cache.
(Each CPU has node_free_pages/16 pages)

and it is used for page table cache.
Then, exit() increase cache, the other hand fork() spent it.

So, if apache type (one parent and many child model) middleware run,
One CPU process fork(), Other CPU process the middleware work and exit().

At that time, One CPU don't have page table cache at all,
Others have maximum caches.

	QList_max = (#ofCPUs - 1) x Free / 16
	=> QList_max / (Free + QList_max) = (#ofCPUs - 1) / (16 + #ofCPUs - 1)

So, How much quicklist spent memory at maximum case?
That is #CPUs proposional because it is per CPU cache but cache amount calculation doesn't use #ofCPUs.

	Above calculation mean

	 Number of CPUs per node            2    4    8   16
	 ==============================  ====================
	 QList_max / (Free + QList_max)   5.8%  16%  30%  48%


Wow! Quicklist can spent about 50% memory at worst case.
More unfortunately, it doesn't have any cache shrinking mechanism.
So it cause some wrong thing.

1. End user misunderstand to memory leak happend.
	=> /proc/meminfo should display amount quicklist

2. It can cause OOM killer
	=> Amount of quicklists shouldn't be proposional to #ofCPUs.

OK, that's a fatal bug and it's present in 2.6.25.x and 2.6.26.x.  A
serious issue.

The patches do apply to both stable kernels and I have tagged them for
backporting into them.  They're nice and small, but I didn't get a
really solid yes-this-is-what-we-should-do from Christoph?


This (from [patch 2/2]): "(Although its patch applied, quicklist can
waste 64GB on 1TB server (= 1TB / 16), it is still too much??)" is a
bit of a worry.  Yes, 64GB is too much!  But at least this is now only
a performance issue rather than a stability issue, yes?

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