Re: [PATCH v2] IB/mlx5: Reduce max order of memory allocated for xlt update

From: Jason Gunthorpe
Date: Wed Mar 31 2021 - 13:53:59 EST


On Thu, Mar 25, 2021 at 11:39:28AM -0300, Jason Gunthorpe wrote:
> On Tue, Mar 23, 2021 at 09:27:38PM -0700, Aruna Ramakrishna wrote:
>
> > > Do you have benchmarks that show the performance of the high order
> > > pages is not relavent? I'm a bit surprised to hear that
> > >
> >
> > I guess my point was more to the effect that an order-8 alloc will
> > fail more often than not, in this flow. For instance, when we were
> > debugging the latency spikes here, this was the typical buddyinfo
> > output on that system:
> >
> > Node 0, zone DMA 0 1 1 2 3 0 1 0 1 1 3
> > Node 0, zone DMA32 7 7 7 6 10 2 6 7 6 2 306
> > Node 0, zone Normal 3390 51354 17574 6556 1586 26 2 1 0 0 0
> > Node 1, zone Normal 11519 23315 23306 9738 73 2 0 1 0 0 0
> >
> > I think this level of fragmentation is pretty normal on long running
> > systems. Here, in the reg_mr flow, the first try (order-8) alloc
> > will probably fail 9 times out of 10 (esp. after the addition of
> > GFP_NORETRY flag), and then as fallback, the code tries to allocate
> > a lower order, and if that too fails, it allocates a page. I think
> > it makes sense to just avoid trying an order-8 alloc here.
>
> But a system like this won't get THPs either, so I'm not sure it is
> relevant. The function was designed as it is to consume a "THP" if it
> is available.

So can we do this with just the addition of __GFP_NORETRY ?

Jason