Re: Delaying writes to disk when there's no need

[Daniel Pittman <daniel@rimspace.net>]

On Mon, 31 Mar 2003, Andrew Morton wrote:
> Daniel Pittman <daniel@rimspace.net> wrote:
>>
>> Capturing a real-time video stream from an IEEE1394 DV stream means
>> writing a stead 3.5MB per second for two on two and a half hours.
>> 
>> Linux isn't great at this, using the default writeout policy, even as
>> recent as 2.5.64. The writer goes OK for a while but, eventually,
>> blocks on writeout for long enough to drop a frame -- more than
>> 8/25ths of a second.
>> 
>> 
>> This can be resolved by tuning the default delay before write-out
>> start to 5 seconds, down from 30, or by running sync every second, or
>> by doing fsync tricks.
> 
> Interesting.
> 
> Yes, I expect that you could fix that up by altering
> dirty_background_ratio and dirty_expire_centisecs.

Those are, in fact, the precise knobs I turned. Well, those and the XFS
pagebuf layer equivalents.

> The problem with fsync() is that it waits on the writeout. You don't
> want that to happen - you just want to tell the kernel "I won't be
> overwriting or deleting this data". Make the kernel queue up and start
> the IO but not wait on its completion.

Yes, that would be good, because then I wouldn't need to write an IPC
thing and fork or thread, so that the second thread can be busy blocking
on the writeout for me.

> It is quite appropriate to do this in fadvise(FADV_DONTNEED) - as a
> lower-latency fsync(). The app would need to call it once per second
> or so.
> 
> It would also throw away any written-back pagecache inside your
> (start, len) which is exactly what your applications wants to happen,
> so the app should be calling fadvise _anyway_.
> 
> What do you think?

I will apply the patch and test later today.  This, however, looks like
a *really* good thing to me.

  Daniel

-- 
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