On Fri, May 4, 2018 at 12:18 AM, Stefan Beller <sbel...@google.com> wrote: > I just measured on git.git and linux.git (both of which are not *huge* by > any standard, but should give a good indication. linux has 6M objects, > and when allocating 1024 at a time, we run into the new block allocation > ~6000 times). > > I could not measure any meaningful difference. > > linux.git $ git count-objects -v > count: 0 > size: 0 > in-pack: 6036543 > packs: 2 > size-pack: 2492985 > prune-packable: 0 > garbage: 0 > size-garbage: 0 > > (with this patch) > Performance counter stats for '/u/git/git count-objects -v' (30 runs): > > 2.123683 task-clock:u (msec) # 0.831 CPUs utilized > ( +- 2.32% ) > 0 context-switches:u # 0.000 K/sec > 0 cpu-migrations:u # 0.000 K/sec > 126 page-faults:u # 0.059 M/sec > ( +- 0.22% ) > 895,900 cycles:u # 0.422 GHz > ( +- 1.40% ) > 976,596 instructions:u # 1.09 insn per cycle > ( +- 0.01% ) > 218,256 branches:u # 102.772 M/sec > ( +- 0.01% ) > 8,331 branch-misses:u # 3.82% of all branches > ( +- 0.61% ) > > 0.002556203 seconds time elapsed > ( +- 2.20% ) > > Performance counter stats for 'git count-objects -v' (30 runs): > > 2.410352 task-clock:u (msec) # 0.801 CPUs utilized > ( +- 2.79% ) > 0 context-switches:u # 0.000 K/sec > 0 cpu-migrations:u # 0.000 K/sec > 131 page-faults:u # 0.054 M/sec > ( +- 0.16% ) > 993,301 cycles:u # 0.412 GHz > ( +- 1.99% ) > 1,087,428 instructions:u # 1.09 insn per cycle > ( +- 0.02% ) > 244,292 branches:u # 101.351 M/sec > ( +- 0.02% ) > 9,264 branch-misses:u # 3.79% of all branches > ( +- 0.57% ) > > 0.003010854 seconds time elapsed > ( +- 2.54% ) > > So I think we could just replace it for now and optimize again later, if it > turns out to be a problem. I think the easiest optimisation is to increase > the allocation size of having a lot more objects per mp_block.
Yeah. I also tested this from a different angle: memory overhead. For 2M objects with one mp_block containing 1024 objects (same setting as alloc.c), the overhead (not counting malloc() internal overhead) is 46KB and we don't have any extra overhead due to padding between objects. This is true for all struct blob, commit, tree and tag. This is really good. alloc.c has zero overhead when measured this way but 46KB is practically zero to me. -- Duy
