linux-kernel - Re: [Linuxarm] Re: [PATCH RFC 0/7] add socket to netdev page frag recycling support

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Date:   Wed, 25 Aug 2021 09:29:06 -0700
From:   David Ahern <dsahern@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Yunsheng Lin <linyunsheng@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Marcin Wojtas <mw@...ihalf.com>, linuxarm@...neuler.org,
        Yisen Zhuang <yisen.zhuang@...wei.com>,
        Salil Mehta <salil.mehta@...wei.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
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        Alexei Starovoitov <ast@...nel.org>,
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        John Fastabend <john.fastabend@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Florian Westphal <fw@...len.de>, xiangxia.m.yue@...il.com,
        linmiaohe <linmiaohe@...wei.com>, Christoph Hellwig <hch@....de>
Subject: Re: [Linuxarm] Re: [PATCH RFC 0/7] add socket to netdev page frag
 recycling support

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