• Linus Torvalds's avatar
    Add specialized object allocator · 855419f7
    Linus Torvalds authored
    This creates a simple specialized object allocator for basic
    This avoids wasting space with malloc overhead (metadata and
    extra alignment), since the specialized allocator knows the
    alignment, and that objects, once allocated, are never freed.
    It also allows us to track some basic statistics about object
    allocations. For example, for the mozilla import, it shows
    object usage as follows:
         blobs:   627629 (14710 kB)
         trees:  1119035 (34969 kB)
       commits:   196423  (8440 kB)
          tags:     1336    (46 kB)
    and the simpler allocator shaves off about 2.5% off the memory
    footprint off a "git-rev-list --all --objects", and is a bit
    faster too.
    [ Side note: this concludes the series of "save memory in object storage".
      The thing is, there simply isn't much more to be saved on the objects.
      Doing "git-rev-list --all --objects" on the mozilla archive has a final
      total RSS of 131498 pages for me: that's about 513MB. Of that, the
      object overhead is now just 56MB, the rest is going somewhere else (put
      another way: the fact that this patch shaves off 2.5% of the total
      memory overhead, considering that objects are now not much more than 10%
      of the total shows how big the wasted space really was: this makes
      object allocations much more memory- and time-efficient).
      I haven't looked at where the rest is, but I suspect the bulk of it is
      just the pack-file loading. It may be that we should pack the tree
      objects separately from the blob objects: for git-rev-list --objects, we
      don't actually ever need to even look at the blobs, but since trees and
      blobs are interspersed in the pack-file, we end up not being dense in
      the tree accesses, so we end up looking at more pages than we strictly
      need to.
      So with a 535MB pack-file, it's entirely possible - even likely - that
      most of the remaining RSS is just the mmap of the pack-file itself. We
      don't need to map in _all_ of it, but we do end up mapping a fair
      amount. ]
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
    Signed-off-by: default avatarJunio C Hamano <junkio@cox.net>
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