Re: [GSoC Patch 0/3] Move generation, graph_pos to a slab

["SZEDER Gábor" <szeder.dev@gmail.com>]

On Mon, Jun 08, 2020 at 11:18:27AM +0530, Abhishek Kumar wrote:
> On Sun, Jun 07, 2020 at 09:53:47PM +0200, SZEDER Gábor wrote:
> > On Thu, Jun 04, 2020 at 10:22:27AM -0400, Derrick Stolee wrote:
> > > On 6/4/2020 3:27 AM, Abhishek Kumar wrote:
> > > > The struct commit is used in many contexts. However, members generation
> > > > and graph_pos are only used for commit-graph related operations and
> > > > otherwise waste memory.
> > > > 
> > > > This wastage would have been more pronounced as transistion to
> > > > generation number v2, which uses 64-bit generation number instead of
> > > > current 32-bits.
> > > 
> > > Thanks! This is an important step, and will already improve
> > > performance in subtle ways.
> > 
> > While the reduced memory footprint of each commit object might improve
> > performance, accessing graph position and generation numbers in a
> > commit-slab is more expensive than direct field accesses in 'struct
> > commit' instances.  Consequently, these patches increase the runtime
> > of 'git merge-base --is-ancestor HEAD~50000 HEAD' in the linux
> > repository from 0.630s to 0.940s.
> > 
> 
> Thank you for checking performance. Performance penalty was something we
> had discussed here [1]. 
> 
> Caching the commit slab results in local variables helped wonderfully in v2 [2].
> For example, the runtime of 'git merge-base --is-ancestor HEAD~50000 HEAD'
> in the linux repository increased from 0.762 to 0.767s. Since this is a
> change of <1%, it is *no longer* a performance regression in my opinion.

Interesting, I measured 0.870s with v2, still a notable increase from
0.630s.