On Feb 20, 2007, at 11:23 AM, Phil Carns wrote:
Robert Latham wrote:
On Tue, Feb 20, 2007 at 07:29:16AM -0500, Phil Carns wrote:
Oh, and one other detail; the memory usage of the servers looks
fine during startup, so this doesn't appear to be a memory leak.
There is quite a bit of CPU work, but I am guessing that is just
berkeley db keeping busy in the iteration function.
How long does it take to scan 1.4 million files on startup?
==rob
That's an interesting issue :)
A few observations:
- we were looking at this on SAN; the results may be different on
local disks
- the db files are on the order of 500 MB for this particular setup
- the time to scan varies depending on if the db files are hot in
the Linux buffer cache
If we start the daemon right after killing another one that just
did the same scan, then the process is CPU intensive, but fast
(about 5 seconds). If we unmount/mount the SAN between the two
runs so that the buffer cache is cleared, then it is very slow
(about 5 minutes).
An interesting trick is to use dd with a healthy buffer size to
read the .db files and throw the output into /dev/null before
starting the servers. This only takes a few seconds, and makes it
so that the scan consistently finishes in just a few seconds as
well. I think the reason is just that it forces the db data into
the Linux buffer cache using an efficient access pattern so that
berkeley db doesn't have to wait on disk latency for whatever small
accesses it is performing.
This seems to indicate that berkeley db's access pattern generated
by PVFS2 for this case isn't very friendly, at least to SANs that
aren't specifically tuned for it.
The 5 minute scan time is a problem, because it makes it hard to
tell when you will actually be able to mount the file system after
the daemons appear to have started. We would be happy to try out
any optimizations here :)
We might try using GET_MULTIPLE for the iterate cases. Hopefully
berkeley uses large block sizes for the reads in that case.
-sam
-Phil
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