pack vs knit based push syd-london packs 1/4 the time of knits... and SFTP write latency glitch?

[Robert Collins]

On Fri, 2007-08-17 at 08:13 +1000, Robert Collins wrote:


> > > 
> > > So Robey, I'm wondering if something is causing glitches with SFTP.
> > 
> > Well, the SFTP code does break up writes into async chunks. So it might
> > still be possible that we are running into latency here. I would expect
> > it not to be. But we do transmit 32kB chunks.
> 
> Thats not enough to counteract the bandwidth-latency multiplier, if we
> are waiting for each chunk to be confirmed before sending the next.

I've tried with a bufsize=128*1024 from home to London:
push pack 128K buffer
real    31m15.595s
user    0m14.729s
sys     0m0.828s

push pack default buffer
real    35m13.149s
user    0m16.197s
sys     0m0.996s

push rsync
real    25m19.660s
user    0m0.184s
sys     0m0.216s

So this says that the ssh channel is able to transfer the data in 25
minutes (possibly better could be done; this isn't about maximal tuning
right now, but just using rsync as a baseline). 

The default buffer in paramiko gives 35 minutes, or 1.4 times longer
than rsync
A 128K buffer gives 31 minutes, or 1.24 times longer than rsync.

Now its nice not to be massive multiples of rsync, but really, we should
be within 10%. I'm going to try an LFTP run later this afternoon, when
I've finished some bulk data transfers I need to do.

But so far I'm pretty solidly convinced that this is a paramiko problem,
not a ssh windowing issue (or rsync would be affected), nor nagle, and
I'm not seeing ECN or tcp window exhaustion during my tests, in fact the
remote end has 16K in its receive buffer consistently available.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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