[dm-devel] Re: [PATCH 0/7] Reduce GFP_ATOMIC allocation failures, candidate fix V3

[Chris Mason <chris mason oracle com>]

On Mon, Nov 16, 2009 at 05:44:07PM +0100, Milan Broz wrote:
> On 11/13/2009 03:46 AM, Chris Mason wrote:
> > On Thu, Nov 12, 2009 at 05:00:05PM -0500, Chris Mason wrote:
> > 
> > [ ...]
> > 
> >>
> >> The punch line is that the btrfs guy thinks we can solve all of this with
> >> just one more thread.  If we change dm-crypt to have a thread dedicated
> >> to sync IO and a thread dedicated to async IO the system should smooth
> >> out.
> 
> Please, can you cc DM maintainers with these kind of patches? dm-devel list at least.
> 

Well, my current patch is a hack.  If I had come up with a proven theory
(hopefully Mel can prove it ;), it definitely would have gone through
the dm-devel lists.

> Note that the crypt requests can be already processed synchronously or asynchronously,
> depending on used crypto module (async it is in the case of some hw acceleration).
> 
> Adding another queue make the situation more complicated and because the crypt
> requests can be queued in crypto layer I am not sure that this solution will help
> in this situation at all.
> (Try to run that with AES-NI acceleration for example.)

The problem is that async threads still imply a kind of ordering.
If there's a fifo serviced by one thread or 10, the latency ramifications
are very similar for a new entry on the list.  We have to wait for a
large portion of the low-prio items in order to service a high prio
item.

With a queue dedicated to sync requests and one dedicated to async,
you'll get better read latencies.  Btrfs has a similar problem around
the crc helper threads and it ends up solving things with two different
lists (high and low prio) processed by one thread.

-chris