Re: [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP

[David Hildenbrand]

On 31.01.24 16:02, Ryan Roberts wrote:
> On 31/01/2024 14:29, David Hildenbrand wrote:
> > > > Note that regarding NUMA effects, I mean when some memory access within the same
> > > > socket is faster/slower even with only a single node. On AMD EPYC that's
> > > > possible, depending on which core you are running and on which memory controller
> > > > the memory you want to access is located. If both are in different quadrants
> > > > IIUC, the access latency will be different.
> > >
> > > I've configured the NUMA to only bring the RAM and CPUs for a single socket
> > > online, so I shouldn't be seeing any of these effects. Anyway, I've been using
> > > the Altra as a secondary because its so much slower than the M2. Let me move
> > > over to it and see if everything looks more straightforward there.
> >
> > Better use a system where people will actually run Linux production workloads
> > on, even if it is slower :)
> >
> > [...]
> >
> > > > > I'll continue to mess around with it until the end of the day. But I'm not
> > > > > making any headway, then I'll change tack; I'll just measure the performance of
> > > > > my contpte changes using your fork/zap stuff as the baseline and post based on
> > > > > that.
> > > >
> > > > You should likely not focus on M2 results. Just pick a representative bare metal
> > > > machine where you get consistent, explainable results.
> > > >
> > > > Nothing in the code is fine-tuned for a particular architecture so far, only
> > > > order-0 handling is kept separate.
> > > >
> > > > BTW: I see the exact same speedups for dontneed that I see for munmap. For
> > > > example, for order-9, it goes from 0.023412s -> 0.009785, so -58%. So I'm
> > > > curious why you see a speedup for munmap but not for dontneed.
> > >
> > > Ugh... ok, coming up.
> >
> > Hopefully you were just staring at the wrong numbers (e.g., only with fork
> > patches). Because both (munmap/pte-dontneed) are using the exact same code path.
>
> Ahh... I'm doing pte-dontneed, which is the only option in your original
> benchmark - it does MADV_DONTNEED one page at a time. It looks like your new
> benchmark has an additional "dontneed" option that does it in one shot. Which
> option are you running? Assuming the latter, I think that explains it.

I temporarily removed that option and then re-added it. Guess you got a 
wrong snapshot of the benchmark :D

pte-dontneed not observing any change is great (no batching possible).

dontneed should hopefully/likely see a speedup.

Great!

--
Cheers,

David / dhildenb