• Hugh Dickins's avatar
    [PATCH] mm: split page table lock · 4c21e2f2
    Hugh Dickins authored
    Christoph Lameter demonstrated very poor scalability on the SGI 512-way, with
    a many-threaded application which concurrently initializes different parts of
    a large anonymous area.
    This patch corrects that, by using a separate spinlock per page table page, to
    guard the page table entries in that page, instead of using the mm's single
    page_table_lock.  (But even then, page_table_lock is still used to guard page
    table allocation, and anon_vma allocation.)
    In this implementation, the spinlock is tucked inside the struct page of the
    page table page: with a BUILD_BUG_ON in case it overflows - which it would in
    the case of 32-bit PA-RISC with spinlock debugging enabled.
    Splitting the lock is not quite for free: another cacheline access.  Ideally,
    I suppose we would use split ptlock only for multi-threaded processes on
    multi-cpu machines; but deciding that dynamically would have its own costs.
    So for now enable it by config, at some number of cpus - since the Kconfig
    language doesn't support inequalities, let preprocessor compare that with
    NR_CPUS.  But I don't think it's worth being user-configurable: for good
    testing of both split and unsplit configs, split now at 4 cpus, and perhaps
    change that to 8 later.
    There is a benefit even for singly threaded processes: kswapd can be attacking
    one part of the mm while another part is busy faulting.
    Signed-off-by: default avatarHugh Dickins <hugh@veritas.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
page_io.c 3.58 KB