Linux-kernel mailing list archive 1998-50,: Re: PATCH: Raw device IO for 2.1.131

# Re: PATCH: Raw device IO for 2.1.131

David S. Miller (davem@dm.cobaltmicro.com)
Sat, 12 Dec 1998 18:54:59 -0800

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From: alan@lxorguk.ukuu.org.uk (Alan Cox)
Date: Sat, 12 Dec 1998 23:32:08 +0000 (GMT)

> - real work. If zero-copy makes a real difference, you'd better
> buy a new
> computer, because your memory subsystem is too slow. Either
> you're IO

Every computer on the planet has a memory subsystem that is too
slow, thats why we have cache. Zero copy makes a difference on real
world networking. Real world file and web serving is about
blasting arbitary blocks of data from memory to and from networks.

I completely agree with Alan.

Linus, if what you want to say is that it's "ok" to have the data go
in and out of the CPU cache for every I/O, and that twiddling block
and inode allocation bits in the filesystem code is "ok" for every
I/O, then you need to have your head seriously examined.

I want 100% of my memory bandwidth, and that means:

1) making the data go once over the memory bus
2) never having the data hit the cpu cache
3) the data path must be user data --> device, no bulky VFS sitting
in the middle dirtying inode and block tables along the way

Why should the hardware people build faster memory subsystems if the
software people are just going to use it half-assedly? Thats what M$
does, but we shouldn't.

Later,
David S. Miller
davem@dm.cobaltmicro.com

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