[Twisted-Python] Re: [Twisted-commits] r10475 - Use writeSequence instead of silly for-loop.
James Y Knight
foom at fuhm.net
Thu Apr 15 13:27:56 EDT 2004
On Apr 5, 2004, at 10:36 AM, Andrew Bennetts wrote:
> Author: spiv
> Date: Mon Apr 5 08:36:43 2004
> New Revision: 10475
> Use writeSequence instead of silly for-loop.
You might not have noticed that writeSequence is implemented like so:
thus negating the performance enhancement recently added by not copying
large data into a new string.
This is of course quite silly, and I suspect a nice performance
enhancement could be achieved by making FileDescriptor.dataBuffer be a
list of strings, rather than a single string. That would both cause
multiple writes in a row to not allocate new strings, and allow
writeSequence to be implemented sanely.
Doesn't look very hard really; looks like you'd only need to touch
write/writeSequence/doWrite. This'd also have the advantage of making
it easier to put a Sendfile-struct in there as well. So pahan's
volunteered to fix this!
Unfortunately, python doesn't expose the writev syscall, which means
that you depend on Nagling to not coalesce the TCP packets. There *is*
a TCP option which you can use to help fix too many small packets:
TCP_CORK on linux and TCP_NOPUSH on FreeBSD. This keeps partial packets
from being sent at all until you close the connection or turn off the
option (except on older freebsd, where it's broken and you need to turn
off the option and call write again to wake it up).
I suspect it's worthwhile to set TCP_CORK/TCP_NOPUSH while the
python-side buffer has any data left in it, or else the kernel may be
deciding to send out small packets when it should be waiting for more
data from an upcoming write call and wasting time/bandwidth. Python has
the TCP_CORK constant, even, yay! FreeBSD will just have to suffer, I
suppose, since python doesn't know about TCP_NOPUSH.
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