[LU-50] pdirops patch for ldiskfs

Related — Tue, 28 Apr 2020 05:45:35 +0000

Changing operations of shared directory are not good even with all SMP improvements, I made some efforts to localize changing operations on a few cores to decrease contention and they can somehow help to improvement performance of pdirops (see my comment on ~~LU-55~~).

At the same time, it's worth to try to increase pdirops performance in ldiskfs level. We had a dynlock based pdirops patch for ldiskfs several years ago, but we didn't maintain it. Actually, I found it's hard to maintain it because it's a complex patch and made a lot of changes to htree code of ext3/ext4. So I'm thinking about to have a more simpler implementation of pdirops patch.

The idea is quite simple (just like how we implement pdirop in ldlm level):

design & implement a advanced lock with multiple modes (CR, PR, CW, PW, EX)
it's just a resource lock resides in osd-ldiskfs & ldiskfs, which will replace current osd_object::oo_ext_idx_sem
we can partially protect resource by locking N hash keys after holding CR & CW
- for CR, sub locking is PR
- for CW, sub locking is PW
for !is_dx(dir), change operations are always holding EX lock on dir, lookup & readdir hold PR lock on dir
for is_dx(dir)
- change operations take CW lock, then take two sub-locks: name-hash as the first key, block number as the second key
- lookup take CR lock, then take two sub-locks: name-hash as the first key, block number as the second key
- readdir take PR lock (it's not necessary to take PR lock, but we already have PR lock in ldlm level, so it doesn't matter)
if we need to split any blocks (name entries block or indexing block) while have CW lock, we release CW lock and take EX lock and retry.
we can disable pdirops ldiskfs by turning osd-ldiskfs pdir_ldiskfs=0/1, if we disabled pdirops for osd-ldiskfs then osd_ldiskfs always EX lock dir on changing and PR lock dir on lookup/readdir.

Although implementation of the lock is a little complex, but we just need very few changes to ext4/ldiskfs and osd-ldiskfs with this lock:

osd-ldiskfs: replace oo_ext_idx_sem with the new lock
ldiskfs: switch lock mode if we need to split/append (in ldiskfs).

PS: I'm thinking about rename parl_lock to htree_lock, because it's really designed for htree operations, and Eric hates confusing naming like "parl_lock"

Whamcloud Community JIRA

[LU-50] pdirops patch for ldiskfs