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    SubjectRe: [rfc][patch] dynamic resizing dentry hash using RCU
    On Fri, Feb 23, 2007 at 05:31:17PM +0100, Eric Dumazet wrote:
    > On Friday 23 February 2007 16:37, Nick Piggin wrote:
    > > The dentry hash uses up 8MB for 1 million entries on my 4GB system is one
    > > of the biggest wasters of memory for me. Because I rarely have more than
    > > one or two hundred thousand dentries. And that's with several kernel trees
    > > worth of entries. Most desktop and probably even many types of servers will
    > > only use a fraction of that.
    > >
    > > So I introduce a new method for resizing hash tables with RCU, and apply
    > > that to the dentry hash.
    > >
    > > The primitive heuristic is that the hash size is doubled when the number of
    > > entries reaches 150% the hash size, and halved when the number is 50%.
    > > It should also be able to shrink under memory pressure, and scale up as
    > > large as we go.
    > >
    > > A pity it uses vmalloc memory for the moment.
    > >
    > > The implementation is not highly stress tested, but it is running now. It
    > > could do a bit more RCU stuff asynchronously rather than with
    > > synchronize_rcu, but who cares, for now.
    > >
    > > The hash is costing me about 256K now, which is a 32x reduction in memory.
    > >
    > > I don't know if it's worthwhile to do this, rather than move things to
    > > other data structures, but something just tempted me to have a go! I'd be
    > > interested to hear comments, and how many holes people can spot in my
    > > design ;)
    > >
    > > Thanks,
    > > Nick
    > Hi Nick
    > Thats a really good idea !
    > The vmalloc() thing could be a problem, so :
    > Could you bring back the support of 'dhash_entries=262144' boot param, so that
    > an admin could set the initial size of dhash table, (and not shrink it under
    > this size even if the number of dentries is low)

    Hi Eric,

    Yeah, that's a good idea. I'll look at doing that.

    > In case dhash_entries is set in boot params, we could try to use
    > alloc_large_system_hash() for the initial table, (eventually using Hugepages
    > (not vmalloc)), if we add a free_large_system_hash() function to be able to
    > free the initial table.
    > Or else, time is to add the possibility for vmalloc() to use hugepages
    > itself...

    That sounds like a nice idea to have a hugepage vmalloc for very large
    allocations. The big NUMA guys already use vmalloc to allocate large hashes,
    so hugepages would probably be a big win for them.
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