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    SubjectRe: [patch] mempool-2.5.1-D2

    On Fri, 14 Dec 2001, Benjamin LaHaise wrote:

    > Btw, wouldn't reservation result in the same effect as these mempools
    > for significantly less code?

    exactly what kind of SLAB based reservation system do you have in mind?
    (what interface, how would it work, etc.) Take a look at how bio.c,
    highmem.c and raid1.c uses the mempool mechanizm, the main properties of
    mempool cannot be expressed via SLAB reservation:

    - mempool allows the use of non-SLAB allocators as the underlying
    allocator. (eg. the highmem.c mempool uses the page allocator to alloc
    lowmem pages. raid1.c uses 4 allocators: kmalloc(), page_alloc(),
    bio_alloc() and mempool_alloc() of a different pool.)

    - mempool_alloc(), if called from a process context, never fails. This
    simplifies lowlevel IO code (which often must not fail) visibly.

    - mempool allows the pooling of arbitrarily complex memory buffers, not
    just a single SLAB buffer. (eg. the raid1.c resync pool uses a
    combination of alloc_mempool(), bio_alloc() and multiple page_alloc()
    buffers. This is also a performance enhancement for raid1.c.)

    - mempool handles allocation in a more deadlock-avoidance-aware way than
    a normal allocator would do:

    - first it ->alloc()'s atomically
    - then it tries to take from the pool if the pool is at least
    half full
    - then it ->alloc()'s non-atomically
    - then it takes from the pool if it's non-empty
    - then it waits for pool elements to be freed

    this makes for five different levels of allocation, ordered for
    performance and blocking-avoidance, while still kicking the VM and
    trying as hard as possible if there is a resource squeeze. In the
    normal case we never touch the mempool spinlocks, we just call
    ->alloc() and if the core allocator does per-CPU caching then we'll
    have the exact same high level of scalability as the underlying

    - mempool adds reservation without increasing the complexity of the
    underlying allocators.


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