Re: order-0 vs order-N driver allocation. Was: [PATCH v10 07/12] net/mlx4_en: add page recycle to prepare rx ring for tx support

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2016-08-04 at 18:19 +0200, Jesper Dangaard Brouer wrote:

> I actually agree, that we should switch to order-0 allocations.
> 
> *BUT* this will cause performance regressions on platforms with
> expensive DMA operations (as they no longer amortize the cost of
> mapping a larger page).


We much prefer reliable behavior, even it it is ~1 % slower than the
super-optimized thing that opens highways for attackers.

Anyway, in most cases pages are re-used, so we only call
dma_sync_single_range_for_cpu(), and there is no way to avoid this.

Using order-0 pages [1] is actually faster, since when we use high-order
pages (multiple frames per 'page') we can not reuse the pages.

[1] I had a local patch to allocate these pages using a very simple
allocator allocating max order (order-10) pages and splitting them into
order-0 ages, in order to lower TLB footprint. But I could not measure a
gain doing so on x86, at least on my lab machines.



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