Re: single copy pipe/fifo

[Manfred Spraul <manfred@colorfullife.com>]

"David S. Miller" wrote:
> 
> Manfred Spraul writes:
>  > * if you run 2 instances on a dual cpu P II/350 it's a big win, but if
>  > you run only one instance, then the bw_pipe processes will jump from one
>  > cpu to the other and it's only a small improvement (~+15%).
> 
> wake_up_interruptible_sync is meant specifically to avoid
> this cpu hopping behavior.
>
I use it whereever possible, but in this case it's not possible:
pipe is empty.

process 1 (on cpu 1)
	write(fd,buf,64kB).
	* creates kiobuf
	* sleeps.

process 2 (on cpu 1)
	read(fd,buf,64kB).
	* reads the data
	* now it must wake up, but it will return from the syscall, thus
wake_up_interruptible().
	* wakeup notices that cpu1 is busy and sends process 1 to cpu 2
	* read returns
	read(fd, buf, 64kB)
	* no data waiting, sleeps.

process 1 (on cpu 2)
	* write returns
	write(fd, buf, 64 kB)
	* creates kiobuf
	* wake_up_interruptible_sync(), since it will schedule
	* schedule()
but now schedule will pull process 2 to cpu 2

I haven't verified the sequence with kdb, but I'm certain that this
happens.

--
	Manfred
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