Re: [Bloat] DSLReports Speed Test has latency measurement built-in

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2015-04-22 at 06:04 +0200, Steinar H. Gunderson wrote:
> On Tue, Apr 21, 2015 at 08:35:21PM +1000, jb wrote:
> > As I understand it (I thought) SO_SNDBUF and SO_RCVBUF are socket buffers
> > for the application layer, they do not change the TCP window size either
> > send or receive.
> 
> I haven't gone into the code and checked, but from practical experience I
> think you're wrong. I've certainly seen positive effects (and verified with
> tcpdump) from reducing SO_SNDBUF on a server that should have no problems at
> all sending data really fast to the kernel.

Well, using SO_SNDBUF disables TCP autotuning.

Doing so :

Pros:

autotuning is known to enable TCP cubic to grow cwnd to bloat levels.
With small enough SO_SNDBUF, you limit this cwnd increase.

Cons:

Long rtt sessions might not have enough packets to utilize bandwidth.


> 
> Then again, this kind of manual tuning trickery got obsolete for me the
> moment sch_fq became available.

Note that I suppose the SO_MAX_PACING rate is really helping you.

Without it, TCP cubic is still allowed to 'fill the pipes' until packet
losses.