Re: [PATCH v6 0/4] fadvise(DONTNEED) support

[Andrea Arcangeli <aarcange@redhat.com>]

Hi Minchan,

On Tue, Feb 22, 2011 at 07:59:51AM +0900, Minchan Kim wrote:
> I don't have a reproducible experiment.
> I started the series with Ben's complain.
> http://marc.info/?l=rsync&m=128885034930933&w=2

Yes I've noticed.

> I don't know Ben could test it with older kernel since recently he got silent.

That's my point, we should check if the "thrashing horribly" is really
a "recently" or if it has always happened before with 2.6.18 and previous.

> I am not sure older kernel worked well such as workload.
> That's because Rik had been pointed out rsync's two touch
> problem(http://linux-mm.org/PageReplacementRequirements) and solved
> part of the problem with remaining half of the active file pages(look
> at inactive_file_is_low) on 2.6.28's big change reclaim.
> So I think the problem about such workload have been in there.

It's possible it's an old problem, but frankly I doubt that any
swapping would have ever happened before, no matter how much rsync you
would run in a loop. As said I also got PM from users asking what they
can do to limit the pagecache because their systems are swapping
overnight because of backup loads, that's definitely not ok. Or at
least there must be a tweak to tell the VM "stop doing the swapping
thing with backup". I didn't yet try to reproduce or debug this as I'm
busy with other compaction/THP related bits.

> And this patch's benefit is not only for preventing working set.
> Before this patch, application should sync the data before fadvise to
> take a effect but it's very inefficient by slow sync operation. If the
> invalidation meet dirty page, it can skip the page. But after this
> patch, application could fdavise without sync because we could move
> the page of head/or tail of inactive list.

I agree, the objective of the patch definitely looks good. Before the
fadvise would be ignored without a fdatasync before it as the pages
were dirty and couldn't be discarded.

> So I think the patch is valuable enough to merge?

The objective looks good, I didn't have time to review all details of
the patches but I'll try to review it later.

> And as I said, I need Ben's help but I am not sure he can.
> Thanks for the review, Andrea.

Thanks for the effort in improving fadvise.

I'd like to try to find the time to check if we've a regression
without fadvise too. It's perfectly ok to use fadvise in rsync but my
point is that the kernel should not lead to trashing of running apps
regardless of fadvise or not, and I think that is higher priority to
fix than the fadvise improvement. But still the fadvise improvement is
very valuable to increase overall performance and hopefully to avoid
wasting most of filesystem cache because of backups (but wasting cache
shouldn't lead to trashing horribly, just more I/O from apps after the
backup run, like after starting the app the first time, trashing
usually means we swapped out or discarded mapped pages too and that's
wrong).

Thanks,
Andrea

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