On Tue, Feb 04, 2020 at 03:58:51PM -0800, Tyler Sanderson wrote:

    >     >
    >     >  1. It is last-resort, which means the system has already gone     through
    >     >     heroics to prevent OOM. Those heroic reclaim efforts are     expensive
    >     >     and impact application performance.
    >
    >     That's *exactly* what "deflate on OOM" suggests.
    >
    >
    > It seems there are some use cases where "deflate on OOM" is desired and
    > others where "deflate on pressure" is desired.
    > This suggests adding a new feature bit "DEFLATE_ON_PRESSURE" that
    > registers the shrinker, and reverting DEFLATE_ON_OOM to use the OOM
    > notifier callback.
    >
    > This lets users configure the balloon for their use case.

    You want the old behavior back, so why should we introduce a new one? Or
    am I missing something? (you did want us to revert to old handling, no?)

Reverting actually doesn't help me because this has been the behavior since
Linux 4.19 which is already widely in use. So my device implementation needs to
handle the shrinker behavior anyways. I started this conversation to ask what
the intended device implementation was.

I think there are reasonable device implementations that would prefer the
shrinker behavior (it turns out that mine doesn't).
For example, an implementation that slowly inflates the balloon for the purpose
of memory overcommit. It might leave the balloon inflated and expect any memory
pressure (including page cache usage) to deflate the balloon as a way to
dynamically right-size the balloon.

So just to make sure we understand, what exactly does your
implementation do?

Two reasons I didn't go with the above implementation:
1. I need to support guests before Linux 4.19 which don't have the shrinker
behavior.
2. Memory in the balloon does not appear as "available" in /proc/meminfo even
though it is freeable. This is confusing to users, but isn't a deal breaker.

If we added a DEFLATE_ON_PRESSURE feature bit that indicated shrinker API
support then that would resolve reason #1 (ideally we would backport the bit to
4.19).

We could declare lack of pagecache pressure with DEFLATE_ON_OOM a
regression and backport the revert but not I think the new
DEFLATE_ON_PRESSURE.

In any case, the shrinker behavior when pressuring page cache is more of an
inefficiency than a bug. It's not clear to me that it necessitates reverting.
If there were/are reasons to be on the shrinker interface then I think those
carry similar weight as the problem itself.
 


    I consider virtio-balloon to this very day a big hack. And I don't see
    it getting better with new config knobs. Having that said, the
    technologies that are candidates to replace it (free page reporting,
    taming the guest page cache, etc.) are still not ready - so we'll have
    to stick with it for now :( .

    >
    > I'm actually not sure how you would safely do memory overcommit without
    > DEFLATE_ON_OOM. So I think it unlocks a huge use case.

    Using better suited technologies that are not ready yet (well, some form
    of free page reporting is available under IBM z already but in a
    proprietary form) ;) Anyhow, I remember that DEFLATE_ON_OOM only makes
    it less likely to crash your guest, but not that you are safe to squeeze
    the last bit out of your guest VM.

Can you elaborate on the danger of DEFLATE_ON_OOM? I haven't seen any problems
in testing but I'd really like to know about the dangers.
Is there a difference in safety between the OOM notifier callback and the
shrinker API?

It's not about dangers as such. It's just that when linux hits OOM
all kind of error paths are being hit, latent bugs start triggering,
latency goes up drastically.

    --
    Thanks,

    David / dhildenb