Re: splice methods in character device driver

From: Jens Axboe
Date: Tue Jun 16 2009 - 07:59:27 EST

On Fri, Jun 12 2009, Steve Rottinger wrote:
> Hi Leon,
> It does seem like a lot of code needs to be executed to move a small
> chunk of data.

It's really not, you should try and benchmark the function call overhead

> Although, I think that most of the overhead that I was experiencing
> came from the cumulative
> overhead of each splice system call. I increased my pipe size using
> Jens' pipe size patch,
> from 16 to 256 pages, and this had a huge effect -- the speed of my
> transfers more than doubled.
> Pipe sizes larger that 256 pages, cause my kernel to crash.

Yes, the system call is more expensive. Increasing the pipe size can
definitely help there.

> I'm doing about 300MB/s to my hardware RAID, running two instances of my
> splice() copy application
> (One on each RAID channel). I would like to combine the two RAID
> channels using a software RAID 0;
> however, splice, even from /dev/zero runs horribly slow to a software
> RAID device. I'd be curious
> to know if anyone else has tried this?

Did you trace it and find out why it was slow? It should not be. Moving
300MB/sec should not be making any machine sweat.

Jens Axboe

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