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[dm-devel] Re: IO scheduler based IO controller V10



On Fri, Sep 25, 2009 at 06:07:24PM +0900, Ryo Tsuruta wrote:
> Hi Vivek,
> 
> Vivek Goyal <vgoyal redhat com> wrote:
> > Higher level solutions are not keeping track of time slices. Time slices will
> > be allocated by CFQ which does not have any idea about grouping. Higher
> > level controller just keeps track of size of IO done at group level and
> > then run either a leaky bucket or token bucket algorithm.
> > 
> > IO throttling is a max BW controller, so it will not even care about what is
> > happening in other group. It will just be concerned with rate of IO in one
> > particular group and if we exceed specified limit, throttle it. So until and
> > unless sequential reader group hits it max bw limit, it will keep sending
> > reads down to CFQ, and CFQ will happily assign 100ms slices to readers.
> > 
> > dm-ioband will not try to choke the high throughput sequential reader group
> > for the slow random reader group because that would just kill the throughput
> > of rotational media. Every sequential reader will run for few ms and then 
> > be throttled and this goes on. Disk will soon be seek bound.
> 
> Because dm-ioband provides faireness in terms of how many IO requests
> are issued or how many bytes are transferred, so this behaviour is to
> be expected. Do you think fairness in terms of IO requests and size is
> not fair?
> 

Hi Ryo,

Fairness in terms of size of IO or number of requests is probably not the
best thing to do on rotational media where seek latencies are significant.

It probably should work just well on media with very low seek latencies
like SSD.

So on rotational media, either you will not provide fairness to random 
readers because they are too slow or you will choke the sequential readers
in other group and also bring down the overall disk throughput.

If you don't decide to choke/throttle sequential reader group for the sake
of random reader in other group then you will not have a good control
on random reader latencies. Because now IO scheduler sees the IO from both
sequential reader as well as random reader and sequential readers have not
been throttled. So the dispatch pattern/time slices will again look like..

	SR1 SR2 SR3 SR4 SR5 RR.....

	instead  of

	SR1 RR SR2 RR SR3 RR SR4 RR ....
 
SR --> sequential reader,  RR --> random reader

> > > >   Buffering at higher layer can delay read requests for more than slice idle
> > > >   period of CFQ (default 8 ms). That means, it is possible that we are waiting
> > > >   for a request from the queue but it is buffered at higher layer and then idle
> > > >   timer will fire. It means that queue will losse its share at the same time
> > > >   overall throughput will be impacted as we lost those 8 ms.
> > > 
> > > That sounds like a bug.
> > > 
> > 
> > Actually this probably is a limitation of higher level controller. It most
> > likely is sitting so high in IO stack that it has no idea what underlying
> > IO scheduler is and what are IO scheduler's policies. So it can't keep up
> > with IO scheduler's policies. Secondly, it might be a low weight group and
> > tokens might not be available fast enough to release the request.
> >
> > > >   Read Vs Write
> > > >   -------------
> > > >   Writes can overwhelm readers hence second level controller FIFO release
> > > >   will run into issue here. If there is a single queue maintained then reads
> > > >   will suffer large latencies. If there separate queues for reads and writes
> > > >   then it will be hard to decide in what ratio to dispatch reads and writes as
> > > >   it is IO scheduler's decision to decide when and how much read/write to
> > > >   dispatch. This is another place where higher level controller will not be in
> > > >   sync with lower level io scheduler and can change the effective policies of
> > > >   underlying io scheduler.
> > > 
> > > The IO schedulers already take care of read-vs-write and already take
> > > care of preventing large writes-starve-reads latencies (or at least,
> > > they're supposed to).
> > 
> > True. Actually this is a limitation of higher level controller. A higher
> > level controller will most likely implement some of kind of queuing/buffering
> > mechanism where it will buffer requeuests when it decides to throttle the
> > group. Now once a fair number read and requests are buffered, and if
> > controller is ready to dispatch some requests from the group, which
> > requests/bio should it dispatch? reads first or writes first or reads and
> > writes in certain ratio?
> 
> The write-starve-reads on dm-ioband, that you pointed out before, was
> not caused by FIFO release, it was caused by IO flow control in
> dm-ioband. When I turned off the flow control, then the read
> throughput was quite improved.

What was flow control doing?

> 
> Now I'm considering separating dm-ioband's internal queue into sync
> and async and giving a certain priority of dispatch to async IOs.

Even if you maintain separate queues for sync and async, in what ratio will
you dispatch reads and writes to underlying layer once fresh tokens become
available to the group and you decide to unthrottle the group.

Whatever policy you adopt for read and write dispatch, it might not match
with policy of underlying IO scheduler because every IO scheduler seems to
have its own way of determining how reads and writes should be dispatched.

Now somebody might start complaining that my job inside the group is not
getting same reader/writer ratio as it was getting outside the group.

Thanks
Vivek


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