question about --bwlimit=

[Wallace Matthews]

I am doing some benchmarking of rsync. I am using the --bwlimit= option to throttle down rsync to predict its operation over slow communications links. I am using rsync 2.6.2 from the release site without any patches. I downloaded the release rather than pull from the CVS tree.

I have 2 servers "wilber" (the remote archive) and "judy" (the local archive) connected with a gig ethernet. I have a file on "judy" that if I use the following command completes in under 1 second:

 time rsync -ar --rsh=rsh bluesAlbums/Pilgrim/track1.mp3 wilber://test/bluesAlbums/Pilgrim 

the track1 file on wilber exists and the track1 file on judy has been touched. The file Track1 is 6.3Meg in size. The checksums file if you do --write-batch is 60K bytes. The difference file is 40k bytes (no differences). I check the modification time on wilber after each transfer to make sure the transfer actually happened.

If I use the command

 time rsync -ar -rsh=rsh --bwlimit=4001 bluesAlbums/Pilgrim/track1.mp3 wilber://test/bluesAlbums/Pilgrim

real = 0.70 to 0.90

If I use the command

 time rsync -ar -rsh=rsh --bwlimit=4000 bluesAlbums/Pilgrim/track1.mp3 wilber://test/bluesAlbums/Pilgrim

real = 1m34.000 to 1m35.000

There are no other processes running on either server, and I touch the file on judy each time I repeat the test. I actually have 2 telnet sessions open on judy and in one I repeat the touch and in the other I repeat the command string after I change the --bwlimit= option. I do this to make sure I don't fat finger things.

I can repeat this time after time. If --bwlimit is > 4000 (ie. 4005, 4025, 4050,5000,7500,10000,100000) real is in the same range as 4001. If --bwlimit is 4000 or under (ie. 3725, 2000, 1000, 100) real is in the same range as 4000.

I can understand bipolar behavior at extremes of bandwidth availability but I cant understand the cutoff being that acute that a 1k difference would yield such a dramatic result. 

At unlimited bandwidth, the real time is twice what it is for rcp so I believe the 0.70 to 0.90 is correct.

??? Is there something going on with --bwlimits around the value of 4000 that could be causing this sharp break.???? 

Wally