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    SubjectRe: missing madvise functionality
    On Thu, 05 Apr 2007 03:31:24 -0400
    Rik van Riel <riel@redhat.com> wrote:

    > Jakub Jelinek wrote:
    >
    > > My guess is that all the page zeroing is pretty expensive as well and
    > > takes significant time, but I haven't profiled it.
    >
    > With the attached patch (Andrew, I'll change the details around
    > if you want - I just wanted something to test now), your test
    > case run time went down considerably.
    >
    > I modified the test case to only run 1000 loops, so it would run
    > a bit faster on my system. I also modified it to use MADV_DONTNEED
    > to zap the pages, instead of the mmap(PROT_NONE) thing you use.
    >

    Interesting...

    Could you please add this patch and see if it helps on your machine ?

    [PATCH] VM : mm_struct's mmap_cache should be close to mmap_sem

    Avoids cache line dirtying : The first cache line of mm_struct is/should_be mostly read.

    In case find_vma() hits the cache, we dont need to access the begining of mm_struct.
    Since we just dirtied mmap_sem, access to its cache line is free.

    In case find_vma() misses the cache, we dont need to dirty the begining of mm_struct.


    Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

    --- linux-2.6.21-rc5/include/linux/sched.h
    +++ linux-2.6.21-rc5-ed/include/linux/sched.h
    @@ -310,7 +310,6 @@ typedef unsigned long mm_counter_t;
    struct mm_struct {
    struct vm_area_struct * mmap; /* list of VMAs */
    struct rb_root mm_rb;
    - struct vm_area_struct * mmap_cache; /* last find_vma result */
    unsigned long (*get_unmapped_area) (struct file *filp,
    unsigned long addr, unsigned long len,
    unsigned long pgoff, unsigned long flags);
    @@ -324,6 +323,7 @@ struct mm_struct {
    atomic_t mm_count; /* How many references to "struct mm_struct" (users count as 1) */
    int map_count; /* number of VMAs */
    struct rw_semaphore mmap_sem;
    + struct vm_area_struct * mmap_cache; /* last find_vma result */
    spinlock_t page_table_lock; /* Protects page tables and some counters */

    struct list_head mmlist; /* List of maybe swapped mm's. These are globally strung


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