Date: Mon, 28 Jun 1999 02:03:41 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: "Daniel J. O'Connor" <darius@dons.net.au> Cc: Zhihui Zhang <zzhang@cs.binghamton.edu>, freebsd-hackers@FreeBSD.ORG, Ladavac Marino <mladavac@metropolitan.at> Subject: Re: RE: Implementation of mmap() in FreeBSD Message-ID: <199906280903.CAA18676@apollo.backplane.com> References: <XFMail.990628181657.darius@dons.net.au>
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:> Otherwise they wouldn't bother to use mmap and instead would use read()
:> and write() :-)
:
:Hmm.. why (unless you forced mmap to use the address you gave it) would it not
:choose the start address to be on a page boundary?
:
:---
:Daniel O'Connor software and network engineer
Because we can't realign the data in the pages without doing a buffer
copy. To force mmap() to align the data to the start of the page requires
it to allocate memory and copy the in-core disk cache to the new memory.
This is extremely wasteful of cpu and memory. The current UNIX mmap
implementation is able to simply map the existing in-core disk cache
directly to the process - no buffer copying is required at all, and
it is extremely memory efficient.
Programmers who use mmap() expect it to be as close to optimal as
possible.
Also, even if we were to realign the data, it would not deal with the
end-of-data case.
-Matt
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