Messages in this thread | | | Date | Sun, 25 Jul 1999 01:41:47 -0600 (MDT) | From | Jason Gunthorpe <> | Subject | Re: real-time threaded IO with low latency (audio) |
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On Sun, 25 Jul 1999, Oliver Xymoron wrote:
> No, I mean spooling to a normal filesystem. Ext2 is fast _on average_. It > will get close to the disk bandwidth on average. > > But realtime is not about averages, it's about the worst case. The worst > case for filesystems can be pretty ugly - there are a lot of data > structures that have to be updated atomically which means there are locks
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.
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
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