On Thu, Jan 30, 2020 at 11:59:46AM -0800, Tyler Sanderson wrote:
>
>
> On Thu, Jan 30, 2020 at 7:31 AM Wang, Wei W <[email protected]> wrote:
>
> On Thursday, January 30, 2020 11:03 PM, David Hildenbrand wrote:
> > On 29.01.20 20:11, Tyler Sanderson wrote:
> > >
> > >
> > > On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <[email protected]
> > > <mailto:[email protected]>> wrote:
> > >
> > > On 29.01.20 01:22, Tyler Sanderson via Virtualization wrote:
> > > > A primary advantage of virtio balloon over other memory reclaim
> > > > mechanisms is that it can pressure the guest's page cache into
> > > shrinking.
> > > >
> > > > However, since the balloon driver changed to using the shrinker
> API
> > > >
> > >
> > <https://github.com/torvalds/linux/commit/71994620bb25a8b109388fefa9
> > e99a28e355255a#diff-fd202acf694d9eba19c8c64da3e480c9> this
> > > > use case has become a bit more tricky. I'm wondering what the
> > intended
> > > > device implementation is.
> > > >
> > > > When inflating the balloon against page cache (i.e. no free
> memory
> > > > remains) vmscan.c will both shrink page cache, but also invoke
> the
> > > > shrinkers -- including the balloon's shrinker. So the balloon
> driver
> > > > allocates memory which requires reclaim, vmscan gets this memory
> > by
> > > > shrinking the balloon, and then the driver adds the memory back
> to
> > the
> > > > balloon. Basically a busy no-op.
>
> Per my understanding, the balloon allocation won’t invoke shrinker as
> __GFP_DIRECT_RECLAIM isn't set, no?
>
> I could be wrong about the mechanism, but the device sees lots of activity on
> the deflate queue. The balloon is being shrunk. And this only starts once all
> free memory is depleted and we're inflating into page cache.
So given this looks like a regression, maybe we should revert the
patch in question 71994620bb25 ("virtio_balloon: replace oom notifier with
shrinker")
Besides, with VIRTIO_BALLOON_F_FREE_PAGE_HINT
shrinker also ignores VIRTIO_BALLOON_F_MUST_TELL_HOST which isn't nice
at all.
So it looks like all this rework introduced more issues than it
addressed ...
I also CC Alex Duyck for an opinion on this.
Alex, what do you use to put pressure on page cache?
>
>
> > > >
> > > > If file IO is ongoing during this balloon inflation then the
> page
> > > cache
> > > > could be growing which further puts "back pressure" on the
> balloon
> > > > trying to inflate. In testing I've seen periods of > 45 seconds
> where
> > > > balloon inflation makes no net forward progress.
>
> I think this is intentional (but could be improved). As inflation does not
> stop when the allocation fails (it simply sleeps for a while and resumes..
> repeat till there are memory to inflate)
> That's why you see no inflation progress for long time under memory
> pressure.
>
> As noted above the deflate queue is active, so it's not just memory allocation
> failures.
>
>
>
>
> Best,
> Wei
>
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