linux-ext4 - [PATCH 0/5] jbd2: Avoid unnecessary locking when buffer is already journaled

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Date:	Thu,  2 Apr 2015 15:58:15 +0200
From:	Jan Kara <jack@...e.cz>
To:	linux-ext4@...r.kernel.org
Cc:	Ted Tso <tytso@....edu>, Jan Kara <jack@...e.cz>
Subject: [PATCH 0/5] jbd2: Avoid unnecessary locking when buffer is already journaled

  Hello,

  this patch set improves do_get_write_access(), jbd2_journal_get_undo_access(),
and jbd2_journal_dirty_metadata() to be completely lockless in case buffer
is already part of an appropriate journalling list. First three patches
are independent small cleanups so they can go in right away I think.

The other two patches *should* improve the situation for frequent bitmap
or inode table block updates. But frankly, I haven't been able to come up
with a load where I'd see significant contention on update of a single buffer
(or it's hidden by a larger lock). Similarly we could see improvements when
do_get_write_access() would be waiting for buffer lock because buffer is
being written out by checkpointing code. But again I wasn't able to hit this
reliably.

Ted, you mentioned at Vault you had a setup where frequent
do_get_write_access() calls were contending in the revoke code. What was the
load exactly? These patches should improve that as well...

								Honza
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