Re: [V4L] Re: v4l2 + select() + read()

[Ben Bridgwater]

Alan Cox wrote:

> > > fact glibc 2.2 has full aio using threads) - you have a whole frame to get
> > > around to the next one plus any internal buffering
> >
> > Sure, and you can get around lack of select() support with threads too. But
> > it's not very convenient or efficient.
>
> What makes you think aio is differently efficient in kernel or user space ?
> In fact the original aio reference implementors took it out of kernel to user
> because they found it made no difference

For a number of reasons:

- IO thread creation and scheduling overhead (could be a lot of them)
- IO thread list processing overhead (driver/controller can maybe do better)
- IO thread to app thread context switch overhead
- IO thread one-at-a-time read/write prevents SCSI TCQ (or other controller)
optimization

Plus probably other low level kernel stuff I don't know about ;-)

Ben




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