Re: [PATCH v3 00/15] Free user PTE page table pages

[Qi Zheng]

On 11/11/21 8:51 PM, David Hildenbrand wrote:

> > > > In the performance test shown on the cover, we repeatedly performed
> > > > touch and madvise(MADV_DONTNEED) actions, which simulated the case
> > > > you said above.
> > > >
> > > > We did find a small amount of performance regression, but I think it is
> > > > acceptable, and no new perf hotspots have been added.
> > >
> > > That test always accesses 2MiB and does it from a single thread. Things
> > > might (IMHO will) look different when only accessing individual pages
> > > and doing the access from one/multiple separate threads (that's what
> >
> > No, it includes multi-threading:
>
> Oh sorry, I totally skipped [2].
>
> > 	while (1) {
> > 		char *c;
> > 		char *start = mmap_area[cpu];
> > 		char *end = mmap_area[cpu] + FAULT_LENGTH;
> > 		pthread_barrier_wait(&barrier);
> > 		//printf("fault into %p-%p\n",start, end);
> >
> > 		for (c = start; c < end; c += PAGE_SIZE)
> > 			*c = 0;
> >
> > 		pthread_barrier_wait(&barrier);
> > 		for (i = 0; cpu==0 && i < num; i++)
> > 			madvise(mmap_area[i], FAULT_LENGTH, MADV_DONTNEED);
> > 		pthread_barrier_wait(&barrier);
> > 	}
> >
> > Thread on cpu0 will use madvise(MADV_DONTNEED) to release the physical
> > memory of threads on other cpu.
>
> I'll have a more detailed look at the benchmark. On a quick glimpse,

Thank you for your time :)

> looks like the threads are also accessing a full 2MiB range, one page at
> a time, and one thread is zapping the whole 2MiB range. A single CPU
> only accesses memory within one 2MiB range IIRC.
>
> Having multiple threads just access individual pages within a single 2
> MiB region, and having one thread zap that memory (e.g., simulate
> swapout) could be another benchmark.

LGTM, I will simulate more scenarios for testing.

> We have to make sure to run with THP disabled (e.g., using
> madvise(MADV_NOHUGEPAGE) on the complete mapping in the benchmark
> eventually), because otherwise you might just be populating+zapping THPs
> if they would otherwise be allowed in the environment.

Yes, I turned off THP during testing:

root@~$ cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]

>

--
Thanks,
Qi