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SubjectRe: [00/41] Large Blocksize Support V7 (adds memmap support)
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mel@skynet.ie (Mel Gorman) writes:

> On (15/09/07 14:14), Goswin von Brederlow didst pronounce:
>> Andrew Morton <akpm@linux-foundation.org> writes:
>>
>> > On Tue, 11 Sep 2007 14:12:26 +0200 Jörn Engel <joern@logfs.org> wrote:
>> >
>> >> While I agree with your concern, those numbers are quite silly. The
>> >> chances of 99.8% of pages being free and the remaining 0.2% being
>> >> perfectly spread across all 2MB large_pages are lower than those of SHA1
>> >> creating a collision.
>> >
>> > Actually it'd be pretty easy to craft an application which allocates seven
>> > pages for pagecache, then one for <something>, then seven for pagecache, then
>> > one for <something>, etc.
>> >
>> > I've had test apps which do that sort of thing accidentally. The result
>> > wasn't pretty.
>>
>> Except that the applications 7 pages are movable and the <something>
>> would have to be unmovable. And then they should not share the same
>> memory region. At least they should never be allowed to interleave in
>> such a pattern on a larger scale.
>>
>
> It is actually really easy to force regions to never share. At the
> moment, there is a fallback list that determines a preference for what
> block to mix.
>
> The reason why this isn't enforced is the cost of moving. On x86 and
> x86_64, a block of interest is usually 2MB or 4MB. Clearing out one of
> those pages to prevent any mixing would be bad enough. On PowerPC, it's
> potentially 16MB. On IA64, it's 1GB.
>
> As this was fragmentation avoidance, not guarantees, the decision was
> made to not strictly enforce the types of pages within a block as the
> cost cannot be made back unless the system was making agressive use of
> large pages. This is not the case with Linux.

I don't say the group should never be mixed. The movable objects could
be moved out on demand. If 64k get allocated then up to 64k get
moved. That would reduce the impact as the kernel does not hang while
it moves 2MB or even 1GB. It also allows objects to be freed and the
space reused in the unmovable and mixed groups. There could also be a
certain number or percentage of mixed groupd be allowed to further
increase the chance of movable objects freeing themself from mixed
groups.

But when you already have say 10% of the ram in mixed groups then it
is a sign the external fragmentation happens and some time should be
spend on moving movable objects.

>> The only way a fragmentation catastroph can be (proovable) avoided is
>> by having so few unmovable objects that size + max waste << ram
>> size. The smaller the better. Allowing movable and unmovable objects
>> to mix means that max waste goes way up. In your example waste would
>> be 7*size. With 2MB uper order limit it would be 511*size.
>>
>> I keep coming back to the fact that movable objects should be moved
>> out of the way for unmovable ones. Anything else just allows
>> fragmentation to build up.
>>
>
> This is easily achieved, just really really expensive because of the
> amount of copying that would have to take place. It would also compel
> that min_free_kbytes be at least one free PAGEBLOCK_NR_PAGES and likely
> MIGRATE_TYPES * PAGEBLOCK_NR_PAGES to reduce excessive copying. That is
> a lot of free memory to keep around which is why fragmentation avoidance
> doesn't do it.

In your sample graphics you had 1152 groups. Reserving a few of those
doesnt sound too bad. And how many migrate types do we talk about. So
far we only had movable and unmovable. I would split unmovable into
short term (caches, I/O pages) and long term (task structures,
dentries). Reserving 6 groups for schort term unmovable and long term
unmovable would be 1% of ram in your situation.

Maybe instead of reserving one could say that you can have up to 6
groups of space not used by unmovable objects before aggressive moving
starts. I don't quite see why you NEED reserving as long as there is
enough space free alltogether in case something needs moving. 1 group
worth of space free might be plenty to move stuff too. Note that all
the virtual pages can be stuffed in every little free space there is
and reassembled by the MMU. There is no space lost there.


But until one tries one can't say.

MfG
Goswin

PS: How do allocations pick groups? Could one use the oldest group
dedicated to each MIGRATE_TYPE? Or lowest address for unmovable and
highest address for movable? Something to better keep the two out of
each other way.
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