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SubjectRe: PATCH: Raw device IO for 2.1.131
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>  - 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. Disk doesnt come into it that much. Sendfile
may be able to be extended to cover some of this, but its only half the needed
API - you need a way to do writev()/readv() type operations with mixed
kernel and user sourced data. Gigabit cards can checksum that sort of stuff
quite happily. Larry seems to have the right idea with his API proposals here.

Think not just about httpd/ftpd which sendfile nicely conquers but also about
NFS/Samba/NCPfs/Netatalk file sharing stuff.

> So I want to be able to do things like frame capture (into a _file_, not a

Frame capture into a file is uninteresting for serious video people because
a) we have this toy computer 2Gig file limit[1] and b) performance is a colossal
issue. a) is easy to work around [apple worked around it nicely with Qt,
intel forgot] b) is a problem. Video people really don't care about throwing
an 80Gig FCAL array at a video problem. No file system who cares - saves fscking

[1] to them anyway.

> buffer cache behaviour under certain circumstances. But basically I still
> see absolutely no reason for supporting raw devices per se.

We need memory locking for stuff like video capture DMA. The demonstration
HDTV chipsets want capture to DMA 1600x1200x24bit data to memory targets.
The existing bttv sick 'vmalloc and look the other way' approach isnt as
good as locking pages for this (Disk I/O issues aside)

Similarly scanners really want to be working direct scatter gather to big
DMA buffers. Its fairly crazy to then go copying those to user space.

I do understand the constraint however - it can't slow down the general
cases or it does become a loss for far too many users.

Alan


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