Re: [PATCH 11/16] rev-list: add bitmap mode to speed up lists

> Vicent Marti <tanoku@gmail.com> writes:
>
>> Calling `git rev-list --use-bitmaps [committish]` is the equivalent
>> of `git rev-list --objects`, but the rev list is performed based on
>> a bitmap result instead of using a manual counting objects phase.
>
> Why would we ever want to not --use-bitmaps, once it actually works?
> I.e., shouldn't this be the default if pack.usebitmaps is set (or
> possibly even core.usebitmaps for these things)?
>
>> These are some example timings for `torvalds/linux`:
>>
>>       $ time ../git/git rev-list --objects master > /dev/null
>>
>>       real    0m25.567s
>>       user    0m25.148s
>>       sys     0m0.384s
>>
>>       $ time ../git/git rev-list --use-bitmaps master > /dev/null
>>
>>       real    0m0.393s
>>       user    0m0.356s
>>       sys     0m0.036s
>
> I see your badass numbers, and raise you a critical issue:
>
>   $ time git rev-list --use-bitmaps --count --left-right origin/pu...origin/next
>   Segmentation fault
>
>   real    0m0.408s
>   user    0m0.383s
>   sys     0m0.022s
>
> It actually seems to be related solely to having negated commits in the
> walk:
>
>   thomas@linux-k42r:~/g(next u+65)$ time git rev-list --use-bitmaps --count origin/pu
>   32315
>
>   real    0m0.041s
>   user    0m0.034s
>   sys     0m0.006s
>   thomas@linux-k42r:~/g(next u+65)$ time git rev-list --use-bitmaps --count origin/pu ^origin/next
>   Segmentation fault
>
>   real    0m0.460s
>   user    0m0.214s
>   sys     0m0.244s
>
> I also can't help noticing that the time spent generating the segfault
> would have sufficed to generate the answer "the old way" as well:
>
>   $ time git rev-list --count --left-right origin/pu...origin/next
>   189     125
>
>   real    0m0.409s
>   user    0m0.386s
>   sys     0m0.022s
>
> Can we use the same trick to speed up merge base computation and then
> --left-right?  The latter is a component of __git_ps1 and can get
> somewhat slow in some cases, so it would be nice to make it really fast,
> too.
>
> --
> Thomas Rast
> trast@{inf,student}.ethz.ch