netdev - Re: SACK compression patch causing performance drop

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Date:   Thu, 8 Nov 2018 07:14:46 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jean-Louis Dupond <jean-louis@...ond.be>, netdev@...r.kernel.org,
        edumazet@...gle.com
Subject: Re: SACK compression patch causing performance drop



On 11/08/2018 12:23 AM, Jean-Louis Dupond wrote:
> Hi,
> 
> Was somebody able to check this?
> Really think this should be fixed :)
> 
> Thanks
> Jean-Louis
> 

I somehow missed this email.

Packet captures might help, please send me

tcpdump -s 128 -i ethX -w sack.pcap

of some samples with or without the sack compression enabled.

> 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
>>