Re: Network performance regression in Linux kernel 6.6 for small socket size test cases

[Bagas Sanjaya]

[also Cc: regressions ML]

On Wed, Feb 28, 2024 at 12:13:27PM +0530, Abdul Anshad Azeez wrote:
> During performance regression workload execution of the Linux
> kernel we observed up to 30% performance decrease in a specific networking
> workload on the 6.6 kernel compared to 6.5 (details below). The regression is
> reproducible in both Linux VMs running on ESXi and bare metal Linux.
> 
> Workload details:
> 
> Benchmark - Netperf TCP_STREAM
> Socket buffer size - 8K
> Message size - 256B
> MTU - 1500B
> Socket option - TCP_NODELAY
> # of STREAMs - 32
> Direction - Uni-Directional Receive
> Duration - 60 Seconds
> NIC - Mellanox Technologies ConnectX-6 Dx EN 100G
> Server Config - Intel(R) Xeon(R) Gold 6348 CPU @ 2.60GHz & 512G Memory
> 
> Bisect between 6.5 and 6.6 kernel concluded that this regression originated
> from the below commit:
> 
> commit - dfa2f0483360d4d6f2324405464c9f281156bd87 (tcp: get rid of
> sysctl_tcp_adv_win_scale)
> Author - Eric Dumazet <edumazet@xxxxxxxxxx>
> Link -
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=
> dfa2f0483360d4d6f2324405464c9f281156bd87
> 
> Performance data for (Linux VM on ESXi):
> Test case - TCP_STREAM_RECV Throughput in Gbps
> (for different socket buffer sizes and with constant message size - 256B):
> 
> Socket buffer size - [LK6.5 vs LK6.6]
> 8K - [8.4 vs 5.9 Gbps]
> 16K - [13.4 vs 10.6 Gbps]
> 32K - [19.1 vs 16.3 Gbps]
> 64K - [19.6 vs 19.7 Gbps]
> Autotune - [19.7 vs 19.6 Gbps]
> 
> >From the above performance data, we can infer that:
> * Regression is specific to lower fixed socket buffer sizes (8K, 16K & 32K).
> * Increasing the socket buffer size gradually decreases the throughput impact.
> * Performance is equal for higher fixed socket size (64K) and Autotune socket
> tests.
> 
> We would like to know if there are any opportunities for optimization in
> the test cases with small socket sizes.
> 

Can you verify the regression on current mainline (v6.8-rc6)?

-- 
An old man doll... just what I always wanted! - Clara

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