Re: [PATCH] dma-buf: system_heap: avoid reclaim for order 4

[T.J. Mercier]

On Tue, Jan 17, 2023 at 10:54 PM John Stultz <jstultz@xxxxxxxxxx> wrote:
>
> On Tue, Jan 17, 2023 at 12:31 AM Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> wrote:
> > > Using order 4 pages would be helpful for many IOMMUs, but it could spend
> > > quite much time in page allocation perspective.
> > >
> > > The order 4 allocation with __GFP_RECLAIM may spend much time in
> > > reclaim and compation logic. __GFP_NORETRY also may affect. These cause
> > > unpredictable delay.
> > >
> > > To get reasonable allocation speed from dma-buf system heap, use
> > > HIGH_ORDER_GFP for order 4 to avoid reclaim.
>
> Thanks for sharing this!
> The case where the allocation gets stuck behind reclaim under pressure
> does sound undesirable, but I'd be a bit hesitant to tweak numbers
> that have been used for a long while (going back to ion) without a bit
> more data.
>
> It might be good to also better understand the tradeoff of potential
> on-going impact to performance from using low order pages when the
> buffer is used.  Do you have any details like or tests that you could
> share to help ensure this won't impact other users?
>
> TJ: Do you have any additional thoughts on this?
>
I don't have any data on how often we hit reclaim for mid order
allocations. That would be interesting to know. However the 70th
percentile of system-wide buffer sizes while running the camera on my
phone is still only 1 page, so it looks like this change would affect
a subset of use-cases.

Wouldn't this change make it less likely to get an order 4 allocation
(under memory pressure)? The commit message makes me think the goal of
the change is to get more of them.

Actually with the low order being 0, I don't think __GFP_COMP makes
sense in LOW_ORDER_GFP. But I guess that flag isn't harmful there.

> thanks
> -john