Date: Wed, 18 Aug 1999 09:42:07 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Luigi Rizzo <luigi@labinfo.iet.unipi.it> Cc: current@FreeBSD.ORG Subject: Re: is pageable memory available in kernel (or will it be ?) Message-ID: <199908181642.JAA47947@apollo.backplane.com> References: <199908181350.PAA12671@labinfo.iet.unipi.it>
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:Hi,
:
:i wonder if some form of pageable memory is available (or will be, or
:can be done with relatively little effort) to pieces of the lower half
:of the kernel. The reason is, the PGM implementation i am working on
:might need to work with really huge windows (megabytes) and on the
:sender side it is not unlikely to have to store a whole file whatever
:its size is.
:
:I suppose sendfile() might have to deal with a similar problem ?
:
: cheers
: luigi
:-----------------------------------+-------------------------------------
: Luigi RIZZO, luigi@iet.unipi.it . Dip. di Ing. dell'Informazione
The kernel supports the notion of 'managed' pages in KVM, but does not
support the notion of actually taking and processing a page fault
from supervisor mode.
sendfile() pre-reserves a block of KVM and stores the reservation
in sf->kva. Essentially, a number of pte entries are being reserved.
The memory area is initially unmapped.
sendfile() then wires the VM pages associated with the file into this
reserved space using pmap_qenter(). It initiates I/O and waits for
it to complete when necessary when pages are not already in the VM
cache, and unwires them when it is through.
DG did a wonderful job writing sendfile, it is very clean code.
The only issue with doing this sort of thing in general is that you
need to reserve a significant amount of KVM. Fortunately we recently
bumped up the amount of KVM in the system so you should be able to
safely reserve a significant amount without any potential problems -
5 to 20MB depending.
-Matt
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