Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Jul 15, 2009 at 02:52:27AM +0800, Vladislav Bolkhovitin wrote:
> 
> Wu Fengguang, on 07/13/2009 04:36 PM wrote:
> >> Test done with XFS on both the target and the initiator. This confirms
> >> your findings, using files instead of block devices is faster, but
> >> only when using the io_context patch.
> > 
> > It shows that the one really matters is the io_context patch,
> > even when context readahead is running. I guess what happened
> > in the tests are:
> > - without readahead (or readahead algorithm failed to do proper
> >   sequential readaheads), the SCST processes will be submitting
> >   small but close to each other IOs.  CFQ relies on the io_context
> >   patch to prevent unnecessary idling.
> > - with proper readahead, the SCST processes will also be submitting
> >   close readahead IOs. For example, one file's 100-102MB pages is
> >   readahead by process A, while its 102-104MB pages may be
> >   readahead by process B. In this case CFQ will also idle waiting
> >   for process A to submit the next IO, but in fact that IO is being
> >   submitted by process B. So the io_context patch is still necessary
> >   even when context readahead is working fine. I guess context
> >   readahead do have the added value of possibly enlarging the IO size
> >   (however this benchmark seems to not very sensitive to IO size).
> 
> Looks like the truth. Although with 2MB RA I expect CFQ to do idling >10 
> times less, which should bring bigger improvement than few %%.
> 
> For how long CFQ idles? For HZ/125, i.e. 8 ms with HZ 250?

Yes, 8ms by default. Note that the 8ms idle time is armed when the
last IO from current process completes. So it would be definitely a
waste if the cooperative process submitted the next read/readahead
IO within this 8ms idle window (without cfq_coop.patch).

Thanks,
Fengguang