high cpu usage of rsync process - sometimes (extremely wired)

[John Van Essen]

On Tue, 7 Feb 2006, Sebastian Schwerdhoefer <sschwerdhoefer at multamedio.de> wrote:
> Hello,
> 
> i have two clients (c1 and c2) with 100% same hard- and software and
> also two servers (s1 and s2) with 100% same hard- and software. The
> servers run "rsync --daemon", the clients start rsync periodically to
> syncronize a directory with them, where c1 syncs with s1 and c2 with
> s2. The directories are the same on s1 and s2!
> 
> Paradoxically the CPU Usage of the rsync process on c1 uses about 50 %
> CPU (as displayed in ps -aux), while rsync on c2 uses about 1 % CPU.

My guess is that c1 is not as busy as c2 inbetween rsyncs, so the disk
cache still has a lot of what it read during the previous rsync.

If so, then c1 does not have to wait for physical disk reads and can
quickly process the list of files it receives from the server (comparing
them with local file info), resulting in high CPU usage and taking a
shorter time to complete.

Otherwise, there may be lots of physical disk I/O required, which will
cause the process to idle more, resulting in lower CPU usage and taking
a longer time to complete.

    John