netdev - Re: SACK compression patch causing performance drop

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Date:   Thu, 8 Nov 2018 09:23:19 +0100
From:   Jean-Louis Dupond <jean-louis@...ond.be>
To:     netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: SACK compression patch causing performance drop

Hi,

Was somebody able to check this?
Really think this should be fixed :)

Thanks
Jean-Louis

On 3/11/18 16:59, Jean-Louis Dupond wrote:
> 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
>