Subject: Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance regression on Xen

From: George Dunlap <[email-masked]>
Date: Wed, 15 Apr 2015 19:04:52 +0100

On 04/15/2015 06:52 PM, Eric Dumazet wrote:
> On Wed, 2015-04-15 at 18:41 +0100, George Dunlap wrote:
> 
>> So you'd be OK with a patch like this?  (With perhaps a better changelog?)
>>
>>  -George
>>
>> ---
>> TSQ: Raise default static TSQ limit
>>
>> A new dynamic TSQ limit was introduced in c/s 605ad7f18 based on the
>> size of actual packets and the amount of data being transmitted.
>> Raise the default static limit to allow that new limit to actually
>> come into effect.
>>
>> This fixes a regression where NICs with large transmit completion
>> times (such as xennet) had a 30% hit unless the user manually tweaked
>> the value in /proc.
>>
>> Signed-off-by: George Dunlap <[email-masked]>
>>
>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>> index 1db253e..8ad7cdf 100644
>> --- a/net/ipv4/tcp_output.c
>> +++ b/net/ipv4/tcp_output.c
>> @@ -50,8 +50,8 @@ int sysctl_tcp_retrans_collapse __read_mostly = 1;
>>   */
>>  int sysctl_tcp_workaround_signed_windows __read_mostly = 0;
>>
>> -/* Default TSQ limit of two TSO segments */
>> -int sysctl_tcp_limit_output_bytes __read_mostly = 131072;
>> +/* Static TSQ limit.  A more dynamic limit is calculated in
>> tcp_write_xmit. */
>> +int sysctl_tcp_limit_output_bytes __read_mostly = 1048576;
>>
>>  /* This limits the percentage of the congestion window which we
>>   * will allow a single TSO frame to consume.  Building TSO frames
>>
> 
> Have you tested this patch on a NIC without GSO/TSO ?
> 
> This would allow more than 500 packets for a single flow.
> 
> Hello bufferbloat.
> 
> So my answer to this patch is a no.

You said:

"I asked you guys to make a test by increasing
sysctl_tcp_limit_output_bytes  You have no need to explain me the code I
wrote, thank you."

Which implies to me that you think you've already pointed us to the
answer you want and we're just not getting it.

Maybe you should stop wasting all of our time and just tell us what
you're thinking.

 -George