On 8/23/21 8:04 AM, Eric Dumazet wrote:

quoted

It seems PAGE_ALLOC_COSTLY_ORDER is mostly related to pcp page, OOM, memory
compact and memory isolation, as the test system has a lot of memory installed
(about 500G, only 3-4G is used), so I used the below patch to test the max
possible performance improvement when making TCP frags twice bigger, and
the performance improvement went from about 30Gbit to 32Gbit for one thread
iperf tcp flow in IOMMU strict mode,

This is encouraging, and means we can do much better.

Even with SKB_FRAG_PAGE_ORDER  set to 4, typical skbs will need 3 mappings

1) One for the headers (in skb->head)
2) Two page frags, because one TSO packet payload is not a nice power-of-two.

interesting observation. I have noticed 17 with the ZC API. That might
explain the less than expected performance bump with iommu strict mode.

The first issue can be addressed using a piece of coherent memory (128
or 256 bytes per entry in TX ring).
Copying the headers can avoid one IOMMU mapping, and improve IOTLB
hits, because all
slots of the TX ring buffer will use one single IOTLB slot.

The second issue can be solved by tweaking a bit
skb_page_frag_refill() to accept an additional parameter
so that the whole skb payload fits in a single order-4 page.