lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Thu, 8 Mar 2012 16:12:21 -0500
From:	Ted Ts'o <>
To:	Jeff Moyer <>
Cc:	Chris Mason <>,
	Boaz Harrosh <>,
	Zach Brown <>, Eric Sandeen <>,,
Subject: Re: [PATCH, RFC] Don't do page stablization if

On Thu, Mar 08, 2012 at 03:42:52PM -0500, Jeff Moyer wrote:
> So now we're back to figuring out how to tell how long I/O will take?
> If writeback is issuing random access I/Os to spinning media, you can
> bet it might be a while.  Today, you could lower nr_requests to some
> obscenely small number to improve worst-case latency.  I thought there
> was some talk about improving the intelligence of writeback in this
> regard, but it's a tough problem, especially given that writeback isn't
> the only cook in the kitchen.

... and it gets worse if there is any kind of I/O prioritization going
on via ionice(), or (as was the case in our example) I/O cgroups were
being used to provide proportional I/O rate controls.  I don't think
it's realistic to assume the writeback code can predict how long I/O
will take when it does a submission.

BTW, I'd have to check (having not looked at the application code in
depth; the bug was primarily solved by bisection and reverting the
problem commit) but I'm not entirely sure the thread doing the write
was calling fsync(); the main issue as I understand things was that
the application wasn't expecting the write(2) system call would block
unexpectedly for long periods of time while doing small buffered,
appending I/O's.  (Again, for the kind of work that distributed
systems do, 99th percentile latency is important!)

						- Ted
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to
More majordomo info at

Powered by blists - more mailing lists