On Sun, 18 Mar 2007 20:08:49 +0000 (GMT) Mel Gorman <mel@csn.ul.ie> wrote:

quoted text
> 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?

CPU load.  From your earlier email I'd decided memory consumption was a
non-issue ;)

quoted text
> 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.


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