• Nick Piggin's avatar
    shrink struct dentry · c2452f32
    Nick Piggin authored
    struct dentry is one of the most critical structures in the kernel. So it's
    sad to see it going neglected.
    
    With CONFIG_PROFILING turned on (which is probably the common case at least
    for distros and kernel developers), sizeof(struct dcache) == 208 here
    (64-bit). This gives 19 objects per slab.
    
    I packed d_mounted into a hole, and took another 4 bytes off the inline
    name length to take the padding out from the end of the structure. This
    shinks it to 200 bytes. I could have gone the other way and increased the
    length to 40, but I'm aiming for a magic number, read on...
    
    I then got rid of the d_cookie pointer. This shrinks it to 192 bytes. Rant:
    why was this ever a good idea? The cookie system should increase its hash
    size or use a tree or something if lookups are a problem. Also the "fast
    dcookie lookups" in oprofile should be moved into the dcookie code -- how
    can oprofile possibly care about the dcookie_mutex? It gets dropped after
    get_dcookie() returns so it can't be providing any sort of protection.
    
    At 192 bytes, 21 objects fit into a 4K page, saving about 3MB on my system
    with ~140 000 entries allocated. 192 is also a multiple of 64, so we get
    nice cacheline alignment on 64 and 32 byte line systems -- any given dentry
    will now require 3 cachelines to touch all fields wheras previously it
    would require 4.
    
    I know the inline name size was chosen quite carefully, however with the
    reduction in cacheline footprint, it should actually be just about as fast
    to do a name lookup for a 36 character name as it was before the patch (and
    faster for other sizes). The memory footprint savings for names which are
    <= 32 or > 36 bytes long should more than make up for the memory cost for
    33-36 byte names.
    
    Performance is a feature...
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    c2452f32
dcache.h 10.9 KB