Determine the best buffer sizes when using socket.send() and socket.recv()

Giampaolo Rodola' gnewsg at gmail.com
Fri Nov 14 20:58:52 CET 2008


On Nov 14, 5:27 pm, Greg Copeland <gtcopel... at gmail.com> wrote:
> On Nov 14, 9:56 am, "Giampaolo Rodola'" <gne... at gmail.com> wrote:
>
>
>
>
>
> > Hi,
> > I'd like to know if there's a way to determine which is the best
> > buffer size to use when you have to send() and recv() some data over
> > the network.
> > I have an FTP server application which, on data channel, uses 8192
> > bytes as buffer for both incoming and outgoing data.
> > Some time ago I received a report from a guy [1] who stated that
> > changing the buffers from 8192 to 4096 results in a drastical speed
> > improvement.
> > I tried to make some tests by using different buffer sizes, from 4 Kb
> > to 256 Kb, but I'm not sure which one to use as default in my
> > application since I noticed they can vary from different OSes.
> > Is there a recommended way to determine the best buffer size to use?
>
> > Thanks in advance
>
> > [1]http://groups.google.com/group/pyftpdlib/browse_thread/thread/f13a82b...
>
> > --- Giampaolohttp://code.google.com/p/pyftpdlib/
>
> As you stated, the answer is obviously OS/stack dependant. Regardless,
> I believe you'll likely find the best answer is between 16K-64K. Once
> you consider the various TCP stack improvements which are now
> available and the rapid increase of available bandwidth, you'll likely
> want to use the largest buffers which do not impose scalability issues
> for your system/application. Unless you have reason to use a smaller
> buffer, use 64K buffers and be done with it. This helps minimize the
> number of context switches and helps ensure the stack always has data
> to keep pumping.
>
> To look at it another way, using 64k buffers requires 1/8 the number
> of system calls and less time actually spent in python code.
>
> If as you say someone actually observed a performance improvement when
> changing from 8k buffers to 4k buffers, it likely has something to do
> with python's buffer allocation overhead but even that seems contrary
> to my expectation. The referenced article was not available to me so I
> was not able to follow and read.
>
> Another possibility is 4k buffers require less fragmentation and is
> likely to perform better on lossy connections. Is it possible he/she
> was testing on a high lossy connection? In short, performance wise,
> TCP stinks on lossy connections.- Hide quoted text -
>
> - Show quoted text -

Thanks for the precious advices.
The discussion I was talking about is this one (sorry for the broken
link, I didn't notice that):
http://groups.google.com/group/pyftpdlib/browse_thread/thread/f13a82b2f41764be#

--- Giampaolo
http://code.google.com/p/pyftpdlib/



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