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

Jason

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