Re: [PATCH] Bias the location of pages freed for min_free_kbytes in the same MAX_ORDER_NR_PAGES blocks

Mel Gorman
2007-03-18T20:08:53Z

On Sun, 18 Mar 2007, Andrew Morton wrote:

> On Sun, 18 Mar 2007 19:05:41 +0000 (GMT) Mel Gorman <mel@csn.ul.ie> wrote:
>
>>> How much additional memory consumption are we expecting here?
>>>
>>
>> Short answer, about 1.5KB on a 1GB system of which 1.3KB is statically
>> defined in the 3 struct zones on a 1 node x86 system.
>>
>> Longer answer that I hopefully have not made any mistakes in - There is
>> the zone overhead which is statically sized and a runtime overhead which
>> depends on the amount of memory in the system. The additional zone
>> overhead is the overhead for additional freelists (larger struct
>> free_area) and is as follows;
>>
>> (MIGRATE_TYPES-1) * sizeof(list_head) * (MAX_ORDER-1)
>>
>> so, on 32 bit in general, thats
>>
>> 4 * 8 * 10 = 320 bytes per zone (would be 240 bytes if MIGRATE_RESERVE is
>> sufficient for higher order allocations
>> instead of MIGRATE_HIGHALLOC)
>>
>> on x86 with DMA, Normal and HighMem, thats 1280 bytes. On a NUMA system,
>> it's 1280 bytes per node. On 64 bit, it would be double because of the
>> larger pointer size. At worst, I guess you are looking at 3KB per node.
>
> That a very modest overhead - not worth the config option, IMO.
>
> The runtime overhead might be a concern - is it possible to quantify
> it?
>

Do you mean performance wise or memory wise?

Memory-wise, something like

===
FLATMEM Case
bits = 0;
for_each_zone(zone) {
bits += (zone->spanned_pages >> (MAX_ORDER-1)) * NR_PAGEBLOCK_BITS);
}
bytes_consumed = bits / 8;

=== SPARSEMEM Case, a rough approximation is
((vm_total_pages * PAGE_SIZE) >> SECTION_SIZE_BITS) * 8

The consumption could be stored in a zone variable similar to
zone->present_pages and visible through /proc/zoneinfo. Would that be
useful?

Performance wise is harder to quantify. There are three places where
issues can show up. The first is with allocation fallbacks where
__rmqueue_fallback() is called. Fallbacks are expensive but fallbacks are
rare except when the zone is too small which is why I probably should be
catching that case explicitly. I used to have a counters patch for
fallbacks. I could bring it up to date to use __count_vm_events() to
quantify fallbacks if you think it would be useful?

The second hotpoint is where the per-cpu lists are searched for a page of
the suitable migrate type. An instruction-level profile on x86 when I
looked at this on x86 showed about 2-4% of the time spent in
get_page_from_freelist() was searching the per-cpu lists for a page of a
suitable type. IIRC, something like 85% of the time there was clearing the
pages although I'd need to double check this to be 100% sure.

The last potential performance hotpoint is where the pageblock flags are
read on every free in get_pageblock_flags_group(). There is probably room
for optimisation there. I haven't an exact quantification available at the
moment but I remember seeing it far down the list of functions time was
spent when I was last looking at this.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab