Re: [RFC PATCH 00/20] Cleanup and optimise the page allocato

Christoph Lameter
2009-02-23T15:43:59Z

On Mon, 23 Feb 2009, Andi Kleen wrote:

> > Counters are surprising expensive, we spent a good chuck of our time in
> > functions like __dec_zone_page_state and __dec_zone_state. In a profiled
> > run of kernbench, the time spent in __dec_zone_state was roughly equal to
> > the combined cost of the rest of the page free path. A quick check showed
> > that almost half of the time in that function is spent on line 233 alone
> > which for me is;
> >
> > (*p)--;
> >
> > That's worth a separate investigation but it might be a case that
> > manipulating int8_t on the machine I was using for profiling is unusually
> > expensive.
>
> What machine was that?
>
> In general I wouldn't expect even on a system with slow char
> operations to be that expensive. It sounds more like a cache miss or a
> cache line bounce. You could possibly confirm by using appropiate
> performance counters.

I have seen similar things occur with some processors. 16 bit or 8 bit
arithmetic can be a problem.

> > Converting this to an int might be faster but the increased
> > memory consumption and cache footprint might be a problem. Opinions?
>
> One possibility would be to move the zone statistics to allocated
> per cpu data. Or perhaps just stop counting per zone at all and
> only count per cpu.

Statistics are in a structure allocated and dedicated for a certain cpu.
It cannot be per cpu data as long as the per cpu allocator has not been
merged. Cache line footprint is reduced with the per cpu allocator.

> > So, by and large it's an improvement of some sort.
>
> That seems like an understatement.

Ack. There is certainly more work to be done on the page allocator. Looks
like a good start to me though.