On 2010-08-09, at 15:53, Darrick J. Wong wrote:

This patch attempts to coordinate barrier requests being sent in by fsync.  Instead of each fsync call initiating its own barrier, there's now a flag to indicate if (0) no barriers are ongoing, (1) we're delaying a short time to collect other fsync threads, or (2) we're actually in-progress on a barrier.

So, if someone calls ext4_sync_file and no barriers are in progress, the flag shifts from 0->1 and the thread delays for 500us to see if there are any other threads that are close behind in ext4_sync_file.  After that wait, the state transitions to 2 and the barrier is issued.  Once that's done, the state goes back to 0 and a completion is signalled.

You shouldn't use a fixed delay for the thread.  500us _seems_ reasonable, if you have a single HDD.  If you have an SSD, or an NVRAM-backed array, then 2000 IOPS is a serious limitation.

What is done in the JBD2 code is to scale the commit sleep interval based on the average commit time.  In fact, the ext4_force_commit-> ...->jbd2_journal_force_commit() call will itself be waiting in the jbd2 code to merge journal commits.  It looks like we are duplicating some of this machinery in ext4_sync_file() already.

It seems like a better idea to have a single piece of code to wait to merge the IOs.  For the non-journal ext4 filesystems it should implement the wait for merges explicitly, otherwise it should defer the wait to jbd2.

Cheers, Andreas