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

[Vladislav Bolkhovitin <vst@vlnb.net>]

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?

> Thanks,
> Fengguang