Path: utzoo!utgpu!watserv1!watdragon!lion!ccplumb
From: ccplumb@lion.waterloo.edu (Colin Plumb)
Newsgroups: comp.arch
Subject: Re: mmap() vs. read() (Was: Re: the Multics from the black lagoon :-))
Message-ID: <20837@watdragon.waterloo.edu>
Date: 13 Feb 90 03:11:05 GMT
References: <8859@portia.Stanford.EDU> <20571@watdragon.waterloo.edu> <1990Feb12.053616.11455@Solbourne.COM> <3556@rti.UUCP> <10468@alice.UUCP> <131682@sun.Eng.Sun.COM> <1990Feb13.003010.23356@utzoo.uucp>
Sender: daemon@watdragon.waterloo.edu
Reply-To: ccplumb@lion.waterloo.edu (Colin Plumb)
Organization: U. of Waterloo, Ontario
Lines: 16

In article <1990Feb13.003010.23356@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>In article <131682@sun.Eng.Sun.COM> gingell@sun.UUCP (Rob Gingell) writes:
>>... At the very least, the read() version will be slower
>>than the mmap() version by the amount of time required to effect the
>>copies from kernel to program buffers...
>
>Assuming your MMU can do copy-on-write, why copy?

Because the odds that the user's buffers are properly aligned are not very
good.  If read() allocated a buffer and returned a pointer to that, they'd
be pretty much the same call.

One thing Mach is showing is that Unix was *not* designed with paging
in mind and you can do all sorts of neat tricks if you have it.
-- 
	-Colin