netdev - SACK compression patch causing performance drop

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Message-ID: <89214271-4496-c438-c4e8-b39f0ae72af7@dupond.be>
Date:   Sat, 3 Nov 2018 16:59:10 +0100
From:   Jean-Louis Dupond <jean-louis@...ond.be>
To:     netdev@...r.kernel.org, edumazet@...gle.com
Subject: SACK compression patch causing performance drop

Hi All,

On recent kernels we noticed a way lower throughput to our SAN system 
than before.
While on pre 4.18 kernels we had 400-700MB/sec read speed, on 4.18+ we 
only had 70-120MB/sec.

The SAN is connected via iSCSI over a 10G network (ixgbe/X520 NICS if it 
matters).

After some debugging, I tried to bisect between 4.17 and 4.18 to see 
what commit caused the slowdown.
It showed that the addition of the SACK compression 
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5d9f4262b7ea41ca9981cc790e37cca6e37c789e) 
was the cause.

And indeed, if I set net.ipv4.tcp_comp_sack_nr to 0 on 4.19 for example, 
the throughput is (almost) back to normal again.
So it seems like this change causes quite some performance issues.

Any ideas?

Thanks
Jean-Louis