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SubjectRe: real-time threaded IO with low latency (audio)
Jason Gunthorpe wrote:
(...)
> Some time ago on the QNX news groups someone was having this problem with
> QNX - they wanted to stream video onto disk at very nearly the full
> bandwidth of the drive (8Meg/sec I think). They found that QNX faultered
> and designed a test case that measured the best/worst/average for a write
> call - turned out the worst case was just awefull - 2-3 times larger then
> their upper limit and much larger than the average case.
>
> I don't know exactly what the QNX developers did, but they actually did
> fix it, they brought the worst case for write down into an acceptable
> region. It was about at this time that their pre-allocation scheme showed
> up in the file system so maybe that had something to do with it.

Pre-allocation sound like a very probably reason to me, it least if the
problem was related to streaming multiple files simultaneously, or even
streaming to a disk containing other files. How to avoid fragmentation
otherwise? (In the case of this kind of applications, a small fragment
is a few hundred kB...)

> They must have some resticitions, a largely unfragemented drive, AV disks
> and only a single application are some likely candiates. But the people
> who needed this simply accepted those limitations - I imagine if you
> wanted to do this on linux you'd start with a dedicated AV drive (no ecc,
> or thermal recalibrations - ment for realtime video streaming) that didn't
> have swap or the root filesystem on it..
>
> Jason


//David

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