Messages in this thread | | | Subject | Re: Delaying writes to disk when there's no need | From | Daniel Pittman <> | Date | Tue, 01 Apr 2003 11:34:50 +1000 |
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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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