This is an increasingly common problem: two layers, both trying to utilise the entire machine by parallelising by the number of CPUs/cores, and so the inner layer gets N² threads/processes. Being quadratic growth, this means that the problem becomes worse the more CPUs you have. With 32 cores, 32² = 1024, though in practice Parallel was told 8 so it’d cap out at only 256 index-pack processes. But that’s a lot of memory needed to support that, especially given that it’s doing no good.
The solution is for only one of the two layers to parallelise.
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Given that you talked about pack.threads, here’s its description from `man git-config`:
> Specifies the number of threads to spawn when searching for best delta matches. This requires that git-pack-objects(1) be compiled with pthreads otherwise this option is ignored with a warning. This is meant to reduce packing time on multiprocessor machines. The required amount of memory for the delta search window is however multiplied by the number of threads. Specifying 0 will cause Git to auto-detect the number of CPUs and set the number of threads accordingly.