Re: -mmX 4G patches feedback [numbers: how much performance impact]

["Martin J. Bligh" <mbligh@aracnet.com>]

>> >> Instead of fiddling with tuning knobs, I'd prefer to just do the UKVA
>> >> idea I've proposed before, and let each process have their own pagetables
>> >> mapped permanently ;-)
>> > 
>> > that will have you pay for pte-highmem even in non-highmem machines.
>> > I'm always been against your above idea ;) It can speedup mmap a bit for
>> > some uncommon case but I believe your slowdown comes from the page faults after
>> > exeve and startup not from mmap with the kernel compile, and worst of
>> > all for non-highmem too (no sysctl or tuning knob can save you then).
>> > Amittedly some mmap intensive workload can get a slight speedup compared
>> > to pte-highmem but I don't think it's common and it has the potential of
>> > slowing down the page faults especially in short lived tasks even w/o
>> > highmem.
>> 
>> You mean the page-faults for the pagetable mappings themselves? I wouldn't
>> have thought that'd make an impact - at least I don't see how it could be
>> worse than pte_highmem. And as we could make it conditional on highmem
> 
> it's worse because you pay for it even with lowmem.
> 
> as for your question for why the overhead is lower on 1/2G boxes, that
> as well is because the probability of the page going into highmem is
> much lower.

Me confused. Are you saying it's worse compared to pte_highmem? or to 
shoving ptes in lowmem?

>> anyway (or even CONFIG_64GB, I'm pretty sure 4GB machines don't need it),
>> I don't think it matters (ie you'd just turn it on instead of pte_highmem).
> 
> 1 single smp kernel with CONFIG64G and ptehighmem=y covers 99% of the
> x86 smp hardware in the market, from 32M of ram to 32G of ram both
> included and always at the 99% of peak possible performance of the
> hardware, that's really nice IMHO, I don't like design solutions that
> requires different kernel image every few gigs of ram you add to the
> machine unless real big gains can be demonstrated.  One can recompile
> and tune as usual, but we should prefer generic design solutions to
> dedicated ones unless they really make an huge difference. Running a
> CONFIG64G with ptehighmem=y on a 512M box may be say 0.1% slower than a
> nohighmem-noptehighmem, Ingo posted the exact PAE vs non-PAE slowdown a
> few days ago, it's non significant.
> 
>> But you're right, we do need to take that into consideration.
> 
> Best really would be to benchmark it, for it I definitely like your
> kernel compile -j benchmark for it (but with mem=800m ;).

Ah. You're worried about the distro situation, where PTE_HIGHMEM would
be turned on for a non-highmem machine, right? Makes more sense I guess.
But runtime switching it probably isn't that hard either ;-)

M.