Date: Sat, 22 Apr 2000 10:06:38 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Alfred Perlstein <bright@wintelcom.net> Cc: Kevin Day <toasty@dragondata.com>, hackers@FreeBSD.ORG Subject: Re: Double buffered cp(1) Message-ID: <200004221706.KAA55294@apollo.backplane.com> References: <200004220255.VAA29433@celery.dragondata.com> <20000422012005.A204@fw.wintelcom.net>
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:extend (using truncate) and then mmap() the destination file, then
:read() directly into the mmap()'d portion.
:
:I'd like to see what numbers you get. :)
:
:--
:-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
Probably not so good considering the number of faults that will
be taken. Also, if the destination filesystem runs out of room
cp will take a random seg-fault trying to access the map. Plus
the dirtying of that many VM pages will seriously effect performance.
read + write is a better way to do it. It is still possible to
double buffer. In this case simply create a small anonymous shared
mmap that fits in the L2 cache (like 128K), setup a pipe, fork, and
have one process read() from the source while the other write()s to the
destination. The added overhead is actually less then 'one buffer copy'
worth if the added buffering fits in the L1 or L2 cache.
-Matt
Matthew Dillon
<dillon@backplane.com>
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