Re: [PATCH] writeback: Per-block device bdi->dirty_writeback_interval and bdi->dirty_expire_interval.

[Kautuk Consul <consul.kautuk@gmail.com>]

Hi Wu,

You're right, the BDI threads should be woken up reliably by the
balance_dirty_pages() and balance_dirty_pages()
needs to be called from all code that is responsible for dirtying the pages.
Sorry, I was not too aware of the balance_dirty_pages() functionality
and the way it was being called in entirety or I would
have spotted this.

Thanks for adding the dirty_background_time into your
over_bground_thresh() formula.

Now that you seem to have included the time into the threshold, I can
relate to your patch better
as a solution for the problems I earlier mentioned.

Thanks again,
Kautuk.


On Fri, Aug 19, 2011 at 7:54 PM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> Hi Kautuk,
>
> On Fri, Aug 19, 2011 at 03:00:30PM +0800, Kautuk Consul wrote:
>> Hi Wu,
>>
>> Yes. I think I do understand your approach.
>>
>> Your aim is to always retain the per BDI timeout value.
>>
>> You want to check for threshholds by mathematically adjusting the
>> background time too
>> into your over_bground_thresh() formula so that your understanding
>> holds true always and also
>> affects the page dirtying scenario I mentioned.
>> This definitely helps and refines this scenario in terms of flushing
>> out of the dirty pages.
>
> Thanks.
>
>> Doubts:
>> i)   Your entire implementation seems to be dependent on someone
>> calling balance_dirty_pages()
>>      directly or indirectly. This function will call the
>> bdi_start_background_writeback() which wakes
>>      up the flusher thread.
>>      What about those page dirtying code paths which might not call
>> balance_dirty_pages ?
>>      Those paths then depend on the BDI thread periodically writing it
>> to disk and then we are again
>>      dependent on the writeback interval.
>>      Can we assume that the kernel will reliably call
>> balance_dirty_pages() whenever the pages
>>      are dirtied ? If that was true, then we would not need bdi
>> periodic writeback threads ever.
>
> Yes. The kernel need a way to limit the total number of dirty pages at
> any given time and to keep them under dirty_ratio/dirty_bytes.
>
> balance_dirty_pages() is such a central place to throttle the dirty
> pages. Whatever code path generating dirty pages are required to call
> into balance_dirty_pages_ratelimited_nr() which will in turn call
> balance_dirty_pages().
>
> So, the values specified by dirty_ratio/dirty_bytes will be executed
> effectively by balance_dirty_pages(). In contrast, the values
> specified by dirty_expire_centisecs is merely a parameter used by
> wb_writeback() to select the eligible inodes to do writeout. The 30s
> dirty expire time is never a guarantee that all inodes/pages dirtied
> before 30s will be timely written to disk. It's better interpreted in
> the opposite way: when under the dirty_background_ratio threshold and
> hence background writeout does not kick in, dirty inodes younger than
> 30s won't be written to disk by the flusher.
>
>> ii)  Even after your rigorous checking, the bdi_writeback_thread()
>> will still do a schedule_timeout()
>>      with the global value. Will your current solution then handle
>> Artem's disk removal scenario ?
>>      Else, you start using your value in the schedule_timeout() call
>> in the bdi_writeback_thread()
>>      function, which brings us back to the interval phenomenon I was
>> talking about.
>
> wb_writeback() will keep running as long as over_bground_thresh().
>
> The flusher will keep writing as long as there are more works, since
> there is a
>
>                if (!list_empty(&bdi->work_list))
>                        continue;
>
> before the schedule_timeout() call.
>
> And the flusher thread will always be woke up timely from
> balance_dirty_pages().
>
> So schedule_timeout() won't block in the way at all.
>
>> Does this patch really help the user control exact time when the write
>> BIO is transferred from the
>> MM to the Block layer assuming balance_dirty_pages() is not called ?
>
> It would be a serious bug if balance_dirty_pages() is somehow not
> called. But note that balance_dirty_pages() is designed to be called
> on every N pages to reduce overheads.
>
> Thanks,
> Fengguang
>

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