Date: Tue, 17 Jan 2006 13:22:37 -0800 From: Julian Elischer <julian@elischer.org> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-hackers@freebsd.org, prime <guomingyan@gmail.com>, kamalp@acm.org Subject: Re: How priority propagation works on read/write lock? Message-ID: <43CD601D.6040807@elischer.org> In-Reply-To: <200601171311.20570.jhb@freebsd.org> References: <1fa17f810601122232l25551bc5n4e4a01ff6b7921e@mail.gmail.com> <200601131422.15208.jhb@freebsd.org> <ac7deb50601152108l15a98197v9eaeaa5a8f0e2d1e@mail.gmail.com> <200601171311.20570.jhb@freebsd.org>
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John Baldwin wrote:
>On Monday 16 January 2006 00:08, Kamal R. Prasad wrote:
>
>
>>you mean, boosting the priority of a reader would be required to avoid
>>priority inversion, but difficult to implement?
>>
>>regards
>>-kamal
>>
>>On 1/14/06, John Baldwin <jhb@freebsd.org> wrote:
>>
>>
>>>I think you just kind of punt and do a best effort. Trying to manage a
>>>list
>>>of current read lock holders would be a bit PITA.
>>>
>>>
>
>Yes. The actual boosting is rather simple, it's keeping track of who has read
>locks that is ugly.
>
>
>
I do wonder if it is worth while.
it would require an internediated structure that would be simultaneously
linked into
a number of structures..
it would be linked into a list of "read locks held by this thread,
and it would be linked into a list of "threads currently reading on this
read lock"
it would however be a rathe small item, and I can imagine that a cache
of a thousand or so of these
would probably do enough for the system.
Something like:
struct rwlock_nexsus {
SLIST_ENTRY( rwlock_nexus) by_thread;
struct thread *owner;
SLIST_ENTRY (rwlock_nexus) by_lock;
struct rwlock *locked;
}
on a x86 this would be 16 bytes long.. on an amd64, 32 bytes
in a page of 4k (x86) you get 256 of them.
that's quite a few considerring that we have only 4 processers or so
running code at a time
and you probably shouldn't be unscheduled while holding one..
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