Deadline for video capture

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Hi all

I suspected that the anticipatory scheduler might not have been the best choice for video capture because of the interruption to writes by reads and the subsequent anticipatory delay associated with it. I have now confirmed that booting with the default anticipatory i/o elevator I get many dropped frames that I don't get if I boot with elevator=deadline.

briefly: dual 7200 rpm ATA5 IDE drives in software RAID0

I guess there isn't really a lot to do about this, it's a compromise one way or the other. The anticipatory scheduler seems better all round but in this large streaming write situation it doesn't seem ideal. Any sysctl settings I could use to blunt the anticipation just before I do video capture?

Con
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echo 0 > /sys/block/*/queue/iosched/antic_expire
Turns it off alltogether.

You could also adjust read and write batch expire settings which are heavily biased toward reads.

Please excuse my potentially very stupid questions....

In AS, are there any sorts of "pressure levels" which indicate how many read and write blocks are pending? Perhaps AS itself could be modified to strongly favor writes once some water mark of available pages (page cache, right?) has been reached.

The main idea behind AS is to increase throughput at the expense of additional latency. It certainly wouldn't hurt us if occasionally there was a sudden burst of well-ordered writes.

I'm assuming that AS attempts to order reads and writes in ascending block number, right? Why not order I/O's by ascending block number regardless of whether they're reads or writes, especially when the write pressure reaches a certain point?

Also, what kind of other system load was going on when the capture was happening? A kernel compile? Is there any feedback from the process scheduler to AS which indicates that an interactive task is what is doing the writes?

It seems to me that video playback would be helped by AS. If capture is hurt by AS, that would be a shame. Who wants to have to reboot or change /proc parameters to switch between record and playback?

But then again, anyone doing professional video editing is not likely to be doing a kernel compile in the background. :)

Basically idle. With the deadline scheduler I was able to continue using the machine without dropping frames.

Con