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SubjectRe: LMbench2.0 results
>> >> Perhaps testing with overcommit on would be useful.
>> >
>> > Well yes - the new overcommit code was a significant hit on the 16ways
>> > was it not? You have some numbers on that?
>>
>> About 20% hit on system time for kernel compiles.
>
> That suprises me a lot. On a 2 way and 4 way the 2.4 memory overcommit
> check code didnt show up. That may be down to the 2 way being on a CPU
> that has no measurable cost for locked operations and the 4 way being an
> ancient ppro a friend has.

Remember this is a NUMA machine - gathering global information
is extremely expensive. On an SMP system, I wouldn't expect it
to show up so much, though it still doesn't seem terribly efficient.
The code is admits it's broken anyway, for the overcommit = 2 case
(which was NOT what I was running - the 20% is for 1). Below is a
simple patch that I've never got around to testing, that I think
will improve that case (not that I'm that interested in setting
overcommit to 2 ;-)).

> If it is the memory overcommit handling then there are plenty of ways to
> deal with it efficiently in the non-preempt case at least. I had
> wondered originally about booking chunks of pages off per CPU (take the
> remaining overcommit divide by four and only when a CPU finds its
> private block is empty take a lock and redistribute the remaining
> allocation). Since boxes almost never get that close to overcommit
> kicking in then it should mean we close to never touch a locked count.

Can you use per-zone stats rather than global ones? That tends to
fix things pretty efficently on these type of machines - per zone
LRUs made a huge impact.

Here's a little patch (untested!). I'll go look at the other case
and see if there's something easy to do, but I think it needs some
significant rework to do anything.

--- virgin-2.5.30.full/mm/mmap.c Thu Aug 1 14:16:05 2002
+++ linux-2.5.30-vm_enough_memory/mm/mmap.c Wed Aug 7 13:26:46 2002
@@ -74,7 +74,6 @@
int vm_enough_memory(long pages)
{
unsigned long free, allowed;
- struct sysinfo i;

atomic_add(pages, &vm_committed_space);

@@ -115,12 +114,7 @@
return 0;
}

- /*
- * FIXME: need to add arch hooks to get the bits we need
- * without this higher overhead crap
- */
- si_meminfo(&i);
- allowed = i.totalram * sysctl_overcommit_ratio / 100;
+ allowed = totalram_pages * sysctl_overcommit_ratio / 100;
allowed += total_swap_pages;

if (atomic_read(&vm_committed_space) < allowed)

-
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