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    SubjectRe: [RFC] frandom - fast random generator module
    On Thu, Oct 16, 2003 at 12:38:28PM -0600, Andreas Dilger wrote:
    > On Oct 16, 2003 12:45 -0500, Matt Mackall wrote:
    > > On Thu, Oct 16, 2003 at 10:20:20AM -0600, Andreas Dilger wrote:
    > > > For Lustre we need a low-cost RNG for generating opaque 64-bit handles in
    > > > the kernel. The use of get_random_bytes() showed up near the top of
    > > > our profiles and we had to invent our own low-cost crappy PRNG instead (it's
    > > > good enough for the time being, but when we start working on real security
    > > > it won't be enough).
    > >
    > > Is this SMP? If so, how many processors? I wonder if you might be
    > > running into some lock contention in the pool entropy transfer -
    > > there's a lock held while mixing new samples into a given pool that
    > > could potentially be a hit.
    >
    > It was a 2-way SMP system. We use the RNG a fair amount (enough to know
    > that 2 CPUs can race and return the same value from get_random_bytes() ;-)

    Sure this is a race and not a birthday paradox? How recent is this?
    Possibly before locking was added to random.c?

    > so we had to put a spinlock around our calls to that. Even so, oprofile
    > showed extract_entropy() and SHATransform() near the top of CPU users.

    Ok, the lock contention would be with add_entropy_words. I've got code
    that reduces calls to SHATransform for /dev/urandom, but it require
    addressing the starvation issues between /dev/random and /dev/urandom first.

    --
    Matt Mackall : http://www.selenic.com : Linux development and consulting
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