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    SubjectRe: [PATCH] Radix-tree pagecache for 2.5

    On 1 Feb 2002, Momchil Velikov wrote:

    > Hmm, worse, yes, the same way as page tables get "worse" with larger
    > address spaces.

    with the difference that for address spaces one of the preferred methods
    of operation is read() [or sendfile(), or any other non-mmap() operation],
    while for pagetables the hardware helps to get locking-free access to the
    mapped contents.

    > Ingo> big files. The thing i'm worried about is the 'big pagecache lock' being
    > Ingo> reintroduced again. If eg. a database application puts lots of data into a
    > Yes, though I'd strongly suspect big database engines can/should/do
    > benefit from doing their application specific caching and indexing,
    > outperforming whatever cache implementation the OS has.

    it's not just databases. It's webservers too, serving content via
    sendfile() from a single, bigger file. Think streaming media servers,
    where the 'movie of the night' sits in a single big binary glob.

    > Ingo> single file (multiple gigabytes - why not), then the
    > mapping-> i_shared_lock becomes a 'big pagecache lock' again, causing
    > Ingo> serious SMP contention for even the read() case. Benchmarks show that it's
    > Ingo> the distribution of locks that matters on big boxes.
    > So, we can use a read-write spinlock instead ->i_shared_lock, ok ?

    using read-write locks does not solve the scalability problem: the problem
    is the bouncing of the spinlock cacheline from CPU to CPU.


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