Re: Balloon pressuring page cache

[David Hildenbrand <david@redhat.com>]

On 29.01.20 20:11, Tyler Sanderson wrote:
> 
> 
> On Wed, Jan 29, 2020 at 2:31 AM David Hildenbrand <david@redhat.com
> <mailto:david@redhat.com>> 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/71994620bb25a8b109388fefa9e99a28e355255a#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.
>     >
>     > 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.
>     >
>     > This wasn't a problem before the change to the shrinker API since
>     forced
>     > balloon deflation only occurred via the OOM notifier callback
>     which was
>     > invoked only after the page cache had depleted.
>     >
>     > Is this new busy behavior working as intended?
> 
>     Please note that the shrinker will only be registered in case we have
>     VIRTIO_BALLOON_F_DEFLATE_ON_OOM - (which is AFAIK very rare) - to
>     implement automatic balloon deflation when the guest is under memory
>     pressure.
> 
> 
>     Are you actually experiencing issues with that or did you just stumble
>     over the code?
> 
> 
> We have a use case that is encountering this (and that registers
> DEFLATE_ON_OOM). We can work around this, but it does seem inefficient.
> I understand there were good reasons for moving away from the OOM
> notifier callback, but I'm wondering if the balloon driver could specify
> a "nice" level to the shrinker API that would cause it to be reclaimed
> from only as a last resort?
>  

Cc-ing linux-mm, Michal and Michael.

Just wondering, how does your workaround look like?

-- 
Thanks,

David / dhildenb