Calvin WanWhile trying to figure out how I was going to set the default parallelism in cw/submodule-status-in-parallel, I noticed some discrepancies between how all the parallelism config options are set in git. I wanted to discuss what we can do now to make them more consistent and also what the standard should be for the future. Here is a list of parallelism config options in git (let me know if I missed any) and how they're set:
grep.threads: if unset or set to 0, git uses number of logical cores. index.threads: if unset or set to 0/true, git uses number of logical cores. If set to 1/false, multithreading is disabled pack.threads: if unset or set to 0, git uses number of logical cores. (documentation doesn't mention what default is) checkout.workers: if unset, defaults to 1. If set to < 1, git uses number of logical cores fetch.parallel: if unset, defaults to 1. If set to 0, git uses number of logical cores (documentation says reasonable default) http.maxRequests: if unset, defaults to 5. If set to < 1, git uses the default 5. submodule.fetchJobs: if unset, defaults to 1. If set to 0, git uses number of logical cores (documentation says reasonable default)
The first inconsistency is the difference in language used to describe when each option is set to "online_cpus()". Some are explicit while others omit it or use language such as "reasonable default". Being explicit for all of the options is probably the easiest documentation fix.
The next inconsistency is for values < 1. Most options use online_cpus() when set to 0 except index.threads which is a special case of its own. Some options error out when set to a negative number while checkout.workers falls back to online_cpus() and http.maxRequests falls back to 5. I don't think we can fix this retroactively unless we decide that all config options will be set to online_cpus() if the value is negative. Should that be the case going forward or should 0 be the only special cased value for now? I can see an argument for allowing other negative values to be configured in the future for different defaulting options.
The final inconsistency is how values are defaulted if unset. Some default to online_cpus() while others default to 1 (http.maxRequests is 5). I want to call out grep.threads specifically here -- on my machine with 48 cores, the default is actually SLOWER than using 1 thread. This is because the grep operation is heavily IO bound, so creating too many threads adds overhead every time the read head changes. Interestingly, this option runs optimally at 4 since that's how many PCIe lanes my SSD uses. While it makes sense to default processor heavy operations to online_cpus(), does it make sense to do the same for IO heavy operations? (I wasn't able to find an equivalent of online_cpus() for drive reading capabilities.) And what about operations that have a fair mix of each?
The safe option is to default to 1 process for many of these config options, but we trade off in improving the experience for the average user that is unaware of these options. If we're already defaulting to online_cpus() for grep.threads and selecting 5 for http.maxRequests, then why not do the same for other options? My suggestion would be defaulting IO dominant operations to min(4, online_cpus()) since that seems like the standard number of lanes for people using SSDs. I would also default operations that have a mix of both to min(8, online_cpus()).
-- Calvin Wan
