Re: [REVIEW] unit number allocation API

From: Julian Elischer (julian_at_elischer.org)
Date: 07/22/04

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    Date: Thu, 22 Jul 2004 10:38:43 -0700
To: Poul-Henning Kamp <phk@freebsd.org>

    Poul-Henning Kamp wrote:

    >We need to allocate unit numbers for (pseudo)devices, and a few
    >places we need to allocate inode numbers for synthetic filesystems
    >(for instance DEVFS).
    >
    >For these applications the overhead of rman(9) can be totally
    >unacceptable (60 bytes per allocation ?) and something more memory
    >frugal is called for.
    >
    >This is a small API I just wrote, targeted specifically for allocating
    >unit numbers and similar spaces.
    >
    >Currently the allocation policy is "lowest free number", but it
    >would be possible to add support for allocating a specific number
    >as well.
    >
    >It uses a mixed run-length/bitmap strategy with fixed size memory
    >chunks (so it can use uma(9) in the kernel).
    >
    >Worst case memory usage is two bits per managed unit-number (worst
    >case is "allocate all units, free all the odd numbered ones").
    >
    >For the typical case where we never free any unit numbers, it will
    >use 52 bytes in total on i386.
    >
    >Please review. (It can be run in userland)
    >
    >Poul-Henning
    >
    >
    >

    please also look at the Thread_id (lwpid) allocator marcel added to
    kern_thread.c

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