Date: Tue, 14 Jan 2003 00:51:07 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Terry Lambert <tlambert2@mindspring.com> Cc: "Alan L. Cox" <alc@imimic.com>, Peter Wemm <peter@wemm.org>, arch@FreeBSD.ORG Subject: Re: getsysfd() patch #1 (Re: Virtual memory question) Message-ID: <200301140851.h0E8p78U078882@apollo.backplane.com> References: <20030114002831.1C8C12A89E@canning.wemm.org> <3E2381F8.85BB90A0@imimic.com> <200301140411.h0E4BgpN078032@apollo.backplane.com> <3E23A086.FC511354@mindspring.com>
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:
:Matthew Dillon wrote:
:>
:> This is a first-attempt workup of getsysfd(). See? I told ya it was
:> trivial!
:
:[ ... ]
:
:What does this uniquely do, which can be done no other way, again?
:
:-- Terry
What Peter asked for (and what this does) is give you a descriptor
that is associated with just a VM Object. You can then mmap() the
descriptor, pass it to other processes and they can mmap() it too.
It's a way of doing swap-backed shared memory without having to deal
with the filesystem. The only other solutions are:
* SysV shared memory, which is not fun to manage and may have weird
size limitations, and has a race condition where a process dying
at just the wrong time may leave a shmem segment lying around in
the kernel.
* A file, which uses the filesystem as backing store. Even with
MAP_NOSYNC a shared file still uses the filesystem as backing store.
This is typically not what is desired.
Another thing I would like to do is descriptor-based timers. So instead
of being limited to just the stupid itimers, or interfering with other
threads/libraries use of [i]timers, you can simply allocate your own by
getting a timer descriptor and then doing cool things with it, like
having it generate a custom signal or selecting on it or kqueue'ing on
it etc... it's something UNIX has needed for a long time actually.
-Matt
Matthew Dillon
<dillon@backplane.com>
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