Re: [PATCH 2/9]: Reduce Log I/O latency

[Matt Mackall <mpm@selenic.com>]

On Thu, Nov 22, 2007 at 12:12:14PM +1100, David Chinner wrote:
> On Thu, Nov 22, 2007 at 01:49:25AM +0100, Andi Kleen wrote:
> > David Chinner <dgc@sgi.com> writes:
> > 
> > > To ensure that log I/O is issued as the highest priority I/O, set
> > > the I/O priority of the log I/O to the highest possible. This will
> > > ensure that log I/O is not held up behind bulk data or other
> > > metadata I/O as delaying log I/O can pause the entire transaction
> > > subsystem. Introduce a new buffer flag to allow us to tag the log
> > > buffers so we can discrimiate when issuing the I/O.
> > 
> > Won't that possible disturb other RT priority users that do not need 
> > log IO (e.g. working on preallocated files)? Seems a little
> > dangerous.
> 
> In all the cases that I know of where ppl are using what could
> be considered real-time I/O (e.g. media environments where they
> do real-time ingest and playout from the same filesystem) the
> real-time ingest processes create the files and do pre-allocation
> before doing their I/O. This I/O can get held up behind another
> process that is not real time that has issued log I/O. 
> 
> Given there is no I/O priority inheritence and having log I/O stall
> will stall the entire filesystem, we cannot allow log I/O to
> stall in real-time environments. Hence it must have the highest
> possible priority to prevent this.

I've seen PVRs that would be upset by this. They put media on one
filesystem and database/apps/swap/etc. on another, but have everything
on a single spindle. Stalling a media filesystem read for a write
anywhere else = fail.

-- 
Mathematics is the supreme nostalgia of our time.