Re: [PATCH 13/35] autonuma: add page structure fields

KOSAKI Motohiro
2012-05-30T09:41:21Z

(5/30/12 5:06 AM), Peter Zijlstra wrote:
> On Wed, 2012-05-30 at 04:25 -0400, KOSAKI Motohiro wrote:
>> (5/29/12 10:54 AM), Peter Zijlstra wrote:
>>> On Tue, 2012-05-29 at 09:56 -0400, Rik van Riel wrote:
>>>> On 05/29/2012 09:16 AM, Peter Zijlstra wrote:
>>>>> On Fri, 2012-05-25 at 19:02 +0200, Andrea Arcangeli wrote:
>>>>
>>>>> 24 bytes per page.. or ~0.6% of memory gone. This is far too great a
>>>>> price to pay.
>>>>>
>>>>> At LSF/MM Rik already suggested you limit the number of pages that can
>>>>> be migrated concurrently and use this to move the extra list_head out of
>>>>> struct page and into a smaller amount of extra structures, reducing the
>>>>> total overhead.
>>>>
>>>> For THP, we should be able to track this NUMA info on a
>>>> 2MB page granularity.
>>>
>>> Yeah, but that's another x86-only feature, _IF_ we're going to do this
>>> it must be done for all archs that have CONFIG_NUMA, thus we're stuck
>>> with 4k (or other base page size).
>>
>> Even if THP=n, we don't need 4k granularity. All modern malloc implementation have
>> per-thread heap (e.g. glibc call it as arena) and it is usually 1-8MB size. So, if
>> it is larger than 2MB, we can always use per-pmd tracking. iow, memory consumption
>> reduce to 1/512.
>
> Yes, and we all know objects allocated in one thread are never shared
> with other threads.. the producer-consumer pattern seems fairly popular
> and will destroy your argument.

THP also strike producer-consumer pattern. But, as far as I know, people haven't observed
significant performance degression. thus I _guessed_ performance critical producer-consumer
pattern is rare. Just guess.

>> My suggestion is, track per-pmd (i.e. 2M size) granularity and fix glibc too (current
>> glibc malloc has dynamically arena size adjusting feature and then it often become
>> less than 2M).
>
> The trouble with making this per pmd is that you then get the false
> sharing per pmd, so if there's shared data on the 2m page you'll not
> know where to put it.
>
> I also know of some folks who did a strict per-cpu allocator based on
> some kernel patches I hope to see posted sometime soon. This because if
> you have many more threads than cpus the wasted space in your areas is
> tremendous.