Messages in this thread | | | Date | Thu, 08 Apr 2004 16:14:08 -0700 | From | "Martin J. Bligh" <> | Subject | Re: -mmX 4G patches feedback [numbers: how much performance impact] |
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>> >> 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.
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