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

[Wu Fengguang]

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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