Re: 2.6.36 io bring the system to its knees

[Sanjoy Mahajan <sanjoy@olin.edu>]

> So this sounds like the backup is just thrashing your cache.

I think it's more than that.  Starting an rxvt shouldn't take 8 seconds,
even with a cold cache.  Actually, it does take a while, so you do have
a point.  I just did

  echo 3 > /proc/sys/vm/drop_caches

and then started rxvt.  That takes about 3 seconds (which seems long,
but I don't know wherein that slowness lies), of which maybe 0.25
seconds is loading and running 'date':

$ time rxvt -e date
real	0m2.782s
user	0m0.148s
sys	0m0.032s

The 8-second delay during the rsync must have at least two causes: (1)
the cache is wiped out, and (2) the rxvt binary cannot be paged in
quickly because the disk is doing lots of other I/O.  

Can the system someknow that paging in the rxvt binary and shared
libraries is interactive I/O, because it was started by an interactive
process, and therefore should take priority over the rsync?

> Does rsync have the option to do an fadvise DONTNEED?

I couldn't find one.  It would be good to have a solution that is
independent of the backup app.  (The 'locate' cron job does a similar
thrashing of the interactive response.)

-Sanjoy

`Until lions have their historians, tales of the hunt shall always
 glorify the hunters.'  --African Proverb

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