On Sun, 18 Mar 2007 20:08:49 +0000 (GMT) Mel Gorman <mel@xxxxxxxxx> wrote:
On Sun, 18 Mar 2007, Andrew Morton wrote:
On Sun, 18 Mar 2007 19:05:41 +0000 (GMT) Mel Gorman <mel@xxxxxxxxx> 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?
CPU load. From your earlier email I'd decided memory consumption was a
non-issue ;)
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.
hm, well. It'd be good to drill down, quantify and, where needed, fix
these things. Because the existence of that config option is quite
undesirabe.