RE: Balloon pressuring page cache

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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 <david@xxxxxxxxxx
> > <mailto:david@xxxxxxxxxx>> 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?


> >     >
> >     > 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.


Best,
Wei




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