[Devel] Re: seems to be a flaw in cfq

[Jens Axboe]

On Tue, Dec 19 2006, Jens Axboe wrote:
> On Tue, Dec 19 2006, Jens Axboe wrote:
> > On Tue, Dec 19 2006, Vasily Tarasov wrote:
> > > ello, Jens.
> > > 
> > > Seems, that we've found some problem in CFQ.
> > > I used your fio tool of version 1.9 to reproduce it.
> > > 2.6.18 vanilla kernel.
> > 
> > I'll look over this report - in the mean time, can you see if the same
> > situation exists in 2.6.19 and 2.6.20-rc1? Would help a lot!
> 
> I just tried to reproduce it in a recent kernel, and it does show
> something very close to what you reported. Using 5 threads, 2 of them
> sharing the same file, the 3 first threads get 6067 -> 6182KiB/sec each,
> and the two threads sharing a file get 975KiB/sec each. That's really
> bizarre, I'll take a good look at this! Thanks for reporting it.

Oh, thinking about this - it could be an artifact of being too fair. The
default is to use O_DIRECT, so the sharing threads probably end up being
blocked waiting for each other to finish the same blocks of io. They
will both be reading from the start of the file to the end, so if they
run alongside each other inside the file, they'll be blocking each other
waiting for io to finish. The same should happen for AS, though. Perhaps
it's bad alias handling in CFQ.

If I add offset=512m to the last thread so that the two sharing threads
read different parts of the file, the result is completely fair
(4.9MiB/sec -> 5.1MiB/sec for each thread).

I'll keep looking.

-- 
Jens Axboe