Re: Streaming disk I/O kills file buffering and makes Linux unusable

[Gerard Roudier <groudier@club-internet.fr>]

On Tue, 24 Aug 1999, Gerard Roudier wrote:

> If you are in a hurry but are a courageous guy ;-), you can, for now, add
> some file control (fcntl) option that tells the kernel to discard the
> pages of data of this file once read by user 

> (or just to set the pages at the end of the LRU in recent kernels),

This suggestion anticipates too much about page map LRU inclusion in the
kernel main stream and just makes the suggestion just broken). Without
Andrea's page LRU, it may happen that useful page of data will still be
thrown away after a shrink_mmap revolution.
By the way, if 2.4 does not include Andrea's page LRU, I know of people 
who will be very disappointed.

> then make appropriate changes in
> the kernel and in the application. The kernel will still prefetch file
> data on read, allowing read streaming, without having to multithread IOs
> at application level.
> 
> Sorry if this has been already suggested.
> 
> Gérard.
> 
> On Mon, 23 Aug 1999, Benno Senoner wrote:
> 
> > On Mon, 23 Aug 1999, Alan Cox wrote:
> > > > I'm afraid, if we want that Linux will be a good multimedia OS, this is a
> > > > _STRONGLY_NEEDED_  feature.
> > > > 
> > > > The user wants to playback his video/audio from disk, and still be able
> > > > to launch his apps, without waiting 10 secs for loading a simple xterm.
> > > > 
> > > > The SCT's  raw-io patches are nice, but not very suitable in a general purpose
> > > > multimedia enviroment, since you can't tell to the user to store his videos on a
> > > > raw partition.
> > > 
> > > Stephens patches are basis of raw I/O on files in a filesystem too.
> > 
> > That sounds nice to me !
> > Any idea when some patches ( for raw file I/O in a filesystem) will be available
> > ? 
> > > 
> > > However what you are saying and raw-io don't neccessarily tally. You are
> > > actually saying "there is a bug in the current page cache handling for
> > > this kind of operation". Far better therefore to fix the heuristic used.
> > 
> > No , I'm not saying that there is a bug, I'm saying that the
> > filebuffering works very well in almost all cases , except of reading large
> > files from disk continuously.
> > This is of course quite logical since the kernel makes assumptions that you
> > will read this large file again, and therefore the kernel has to put it into
> > the buffer cache.
> > 
> > In this case I don't need raw I/O to get fast response or higher throughput,
> > but only  to avoid that the streaming apps do not monopolizes the buffer, which
> > is very bad.
> > 
> > will the SCT's patches add an O_DIRECT -like flag ala SGI ?
> > 
> > regards,
> > Benno.
> > 
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>