On Mar 1, 2007, at 10:00 AM, Sam Lang wrote:
On Mar 1, 2007, at 9:52 AM, Phil Carns wrote:
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.
I couldn't find anything in the berkeley db about DB_MULTIPLE_KEY
and DB_PREV not being allowed, but when tried it returns an error
about Illegal flag combinations.
I guess the doc does say its not allowed:
------
The DB_MULTIPLE_KEY flag may only be used with the DB_CURRENT,
DB_FIRST, DB_GET_BOTH, DB_GET_BOTH_RANGE, DB_NEXT, DB_NEXT_DUP,
DB_NEXT_NODUP, DB_SET, DB_SET_RANGE, and DB_SET_RECNO options.
------
It seems strange that they have that restriction. Most of the btree
implementation seems symmetric except for that.
-sam
So our option is to either use DB_PREV without DB_MULTIPLE (no
storage format changes), or change the comparison function and
storage format so that we can use DB_NEXT with DB_MULTIPLE_KEY.
Checking the storage format version and providing the appropriate
comparison function wouldn't be hard though, and wouldn't require
any "migration" of the old to new format. Older formats wouldn't
benefit from the performance improvements though.
-sam
-Phil
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