Sam Lang wrote:
On Feb 28, 2007, at 6:54 AM, Phil Carns wrote:
I know that you guys still have some ongoing discussion about the long
range design for tracking handles, but I have another item about the
current implementation that might be of interest.
Most of the remaining startup performance problem (after Sam's
optimization patches) appears to be a result of how the db is ordered.
If I modify the attr db's comparison function so that it has a "<"
rather than ">", then all of the preads during startup go in order
through the db rather than backwards. This takes the startup time on a
cold db down to just 34 seconds. Previously it was 2 minutes 22
seconds.
It still could be faster, but that seems to be the biggest part of the
time. I imagine the rest of it is just the access size (4 KB at a
time) that might be tunable through some berkeley db settings.
The downside of making that particular change to the comparison
method is that it breaks storage space compatibility.
I wonder if it might be possible to accomplish the same thing in the
current db format by modifying iterate_handles() to just run the cursor
backwards (using DB_PREV instead of DB_NEXT)? That wouldn't hurt
storage space compability (if it works), but I don't know if it makes
any difference to callers of that function what order the handles
come out in.
It doesn't matter to the caller. You'll also need to set the cursor to
the last position in the db with DB_LAST. Does DB_PREV work with
DB_MULTIPLE though? Its not clear from the above, does the improvement
to 34 seconds occur with MULTIPLE or without?
I mentioned previously that the dspace db gets opened with the RECNUM
flag. I don't think that's necessary, and removing it will invariably
improve performance, but we need a way to return the position for
iterate_handles. The easiest thing to do is turn PVFS_ds_position into
a uint64_t (currently its only uint32_t). That breaks interfaces and
protocols though.
I don't know if the PREV approach would work with MULTIPLE or not. The
34 second times (with inverted comparison function) were run with your
MULTIPLE patches applied. I didn't try it without the patches.
-Phil
_______________________________________________
Pvfs2-developers mailing list
Pvfs2-developers@beowulf-underground.org
http://www.beowulf-underground.org/mailman/listinfo/pvfs2-developers
