[Bug 1337] SCP performance twice as slow as RCP

bugzilla-daemon at bugzilla.mindrot.org bugzilla-daemon at bugzilla.mindrot.org
Tue Jul 10 22:59:30 EST 2007


--- Comment #1 from Darren Tucker <dtucker at zip.com.au>  2007-07-10 22:59:27 ---
Because scp does fundamentally more work (encryption,mac) and sends
more data (the mac) you probably won't get scp as fast as rcp as long
as those remain.  Given enough CPU horsepower, in theory you might be
able to get close, though.

There's a few things that you can do: for any version of OpenSSH you
should choose a cipher and mac that is fast on your hardware.  You can
test them with the "openssl speed" tool.  Unless you have odd hardware
or crypto accelerators, this is likely to be arcfour and hmac-md5-96.

There are also some speed improvements in the current development
version that haven't made a release yet:
1) channel window size increase. Only likely to help if you have a
long, fat pipe.
2) a new, faster MAC: umac-64 at openssh.com (see
3) MAC contexts are cleared and reused rather than
deallocating/reallocating them, saving ~10% of CPU time.

There's another change pending that has not gone in yet:
4) increasing the scp buffer size and polling rather than busy-waiting
in scp when writes stall.  This is waiting for some slacker to review
and test (oh, wait, that would be me :-).

#1 and #4 are somewhat similar to the changes in the hpn-ssh patch at
PSC, although different in implementation. 

Normally I would just ask you to try a snapshot from ftp.openbsd.org
but they seem to be stale at the moment.  If you would like to try it
out I have put up a temporary snap at
http://www.zip.com.au/~dtucker/tmp/openssh-SNAP_DT-20070710.tar.gz . 
If you do try it, please let us know how it goes!

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