[Ocfs2-devel] Ocfs2 performance bugs of doom

[Daniel Phillips]

Andrew Morton wrote:
> Daniel Phillips <phillips at google.com> wrote:
>>   	assert_spin_locked(&dlm->spinlock);
>> +	bucket = dlm->lockres_hash + full_name_hash(name, len) % DLM_HASH_BUCKETS;
>>
>> -	hash = full_name_hash(name, len);
>
> err, you might want to calculate that hash outside the spinlock.

Yah.

> Maybe have a lock per bucket, too.

So the lock memory is as much as the hash table? ;-)

> A 1MB hashtable is verging on comical.  How may data are there in total?

Even with the 256K entry hash table, __dlm_lookup_lockres is still the
top systime gobbler:

-------------
real 31.01
user 25.29
sys 3.09
-------------

CPU: P4 / Xeon, speed 2793.37 MHz (estimated)
Counted GLOBAL_POWER_EVENTS events (time during which processor is not stopped) with a unit mask of 0x01 (mandatory) count 240000
samples  %        image name               app name                 symbol name
-------------------------------------------------------------------------------
17071831 71.2700  libbz2.so.1.0.2          libbz2.so.1.0.2          (no symbols)
   17071831 100.000  libbz2.so.1.0.2          libbz2.so.1.0.2          (no symbols) [self]
-------------------------------------------------------------------------------
2638066  11.0132  vmlinux                  vmlinux                  __dlm_lookup_lockres
   2638066  100.000  vmlinux                  vmlinux                  __dlm_lookup_lockres [self]
-------------------------------------------------------------------------------
332683    1.3889  oprofiled                oprofiled                (no symbols)
   332683   100.000  oprofiled                oprofiled                (no symbols) [self]
-------------------------------------------------------------------------------
254736    1.0634  vmlinux                  vmlinux                  ocfs2_local_alloc_count_bits
   254736   100.000  vmlinux                  vmlinux                  ocfs2_local_alloc_count_bits [self]
-------------------------------------------------------------------------------
176794    0.7381  tar                      tar                      (no symbols)
   176794   100.000  tar                      tar                      (no symbols) [self]
-------------------------------------------------------------------------------

Note, this is uniprocessor, single node on a local disk.  Something
pretty badly broken all right.  Tomorrow I will take a look at the hash
distribution and see what's up.

I guess there are about 250k symbols in the table before purging
finally kicks in, which happens 5th or 6th time I untar a kernel tree.
So, 20,000 names times 5-6 times the three locks per inode Mark
mentioned.  I'll actually measure that tomorrow instead of inferring
it.

I think this table is per-ocfs2-mount, and really really, a meg is
nothing if it makes CPU cycles  go away.  That's .05% of the memory
on this box, which is a small box where clusters are concerned.  But
there is also some gratuitous cpu suck still happening in there that
needs investigating.  I would not be surprised at all to learn that
full_name_hash is a terrible hash function.

Regards,

Daniel