> frames per second. This means a sustained rate of roughly 10MB per
> second. Lossy compression would falsify the results and is thus not
Thats about the bandwidth of a disk
Yes. So on a perfect real-time operating system this would be
entirely sufficient for monochrome operation.
> Now how to do the recording? The easiest solution would probably be
> memory-mapping a recording file and doing a single large read-call
> from the framegrabber (video4linux device) to the file.
Or memory map the frame grabber and write to the file - thats how the
existing video4linux rtjpeg tools work
Well, if the frame grabber driver does not mind mapping 20GB in one
piece... I have to test both and see what I will do.
> File size: the ext2 file system tops out at 2GB file size, which is
Not on a 64bit machine like the Alpha
Well, if it does not, it is 2.2.x news. On our stock RedHat 5.2
Alpha, a write via dd if=/dev/zero is aborted after 2GB, with an
appropriate error message. Of course I will be glad to hear of any
change made. I would guess that >2GB files on the Alpha are possible
on NFS and some foreign file systems.
> Efficiency of memory map: will all that happens in that scenario be a
> transfer from framegrabber to memory in one copy done in busmaster
> mode by the framegrabber, and from memory to disk in another busmaster
> done by the SCSI controller? Is there even a possibility that a
Right now you can set it up so that you DMA from the capture card to
the memory and then write it ought through the buffer cache. Stephen
Tweedie has O_DIRECT patches for Linux (I dont know how tested they are
on Alpha but they survive Oracle on x86 so ..) that willl mean you are
doing DMA direct back out from the same memory
Sounds good.
Thank you,
David Kastrup Phone: +49-234-700-5570
Email: dak@neuroinformatik.ruhr-uni-bochum.de Fax: +49-234-709-4209
Institut für Neuroinformatik, Universitätsstr. 150, 44780 Bochum, Germany
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