netdev - Re: [PATCH v1 net-next 1/5] Documentations: Analyze heavily used Networking related structs

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Date: Sat, 16 Sep 2023 16:36:37 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Coco Li <lixiaoyan@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Mubashir Adnan Qureshi <mubashirq@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
	Chao Wu <wwchao@...gle.com>, Wei Wang <weiwan@...gle.com>
Subject: Re: [PATCH v1 net-next 1/5] Documentations: Analyze heavily used
 Networking related structs

On Sat, Sep 16, 2023 at 01:06:21AM +0000, Coco Li wrote:
> Analyzed a few structs in the networking stack by looking at variables
> within them that are used in the TCP/IP fast path.
> 
> Fast path is defined as TCP path where data is transferred from sender to
> receiver unidirectionaly. It doesn't include phases other than
> TCP_ESTABLISHED, nor does it look at error paths.
> 
> We hope to re-organizing variables that span many cachelines whose fast
> path variables are also spread out, and this document can help future
> developers keep networking fast path cachelines small.
> 
> Optimized_cacheline field is computed as
> (Fastpath_Bytes/L3_cacheline_size_x86), and not the actual organized
> results (see patches to come for these).

What value do you use for L3_cacheline_size_x86? What is
L3_cacheline_size_arm64, L3_cacheline_size_s390, etc.

Do you have any profile data which compares L3 cache misses vs L2, vs
L1. I guess there should be some gains by changing the order of
structure members, such that those which are used at a similar time
are in the same L1 and L2 cache lines, and so only need to be fetched
once, so reducing cache thrashing.

      Andrew