[wplug] compression

Michael Semcheski lists at immuneit.com
Thu Apr 14 19:52:46 EDT 2005


Patrick Wagstrom wrote:
> The -C flag will tell SSH/SCP to compress stuff.  In general I use it
>  all the time.  CPUs are so much faster than networks still so 
> compression is your friend.

While I agree that compression is good on a slow network, this is not
true for fast networks.  As a test, I took a 200Mb
quicktime file and sent it across my home 100Mb/fd network via scp with
and without compression.  I later tried gziping this file (same
compression that ssh uses) and it compressed about 4%.

Without compression, it took 23 seconds (8MB/sec).
With compression, it took roughly 1 minute and 20 seconds (2.1MB/sec).

So lzw compression does not do a lot for movie files.  Everybody knew
that.  My next test I took about 250Mb of tar'd php applications (so
there were some image files, but it was mostly text).  Gzip was able to
compress it about 60%, (though I didn't use the gzipped file in my tests).

Without compression, it took 28 seconds (8MB/sec).
With compression, it took about 50 seconds (4.7MB/sec).

So compression over the 100Mb network slowed things considerably.

The thing that surprised me was that I noticed that with compression, my
CPU usage (Centrino) went up to 50% for most of the transfer.  Without
compression, it was around 80% for most of the transfer.

This is not the fastest processor on the market, but its well over 1Ghz.
 Even still, it could not keep up with the network.

Mike



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