Comment by marcosdumay
1 day ago
CoW is probably a good idea whether you use fork or not. Or rather, fork is probably a better option than just exec exactly because it can benefit from CoW.
At least on systems with virtual addressing. If you want to go into physical addressing, then yes, maybe it's a problem. But Linux will never touch anything with physical addressing, so I don't see what people are complaining about.
CoW is probably a good idea regardless, yeah. Overcommit is more questionable. Regardless, both ought to be argued based on their own merits. It's unfortunate that both are necessary as a consequence of fork().
I don't think fork() mandates overcommit. OpenBSD doesn't seem to even allow overcommit or have an OOM killer, memory allocations that exceed available capacity fail immediately even if the memory is not touched.
Let's say you have 1GB RAM. You're running program that occupies 600 MB. Now this program wants to launch second small program that occupies 1 MB.
You're doing fork + exec.
If you're overcommiting, fork will not reserve another 600 MB, and exec immediately after fork will cause total system usage to be 601 MB.
If you're not overcommiting, that fork will fail, because total memory consumption will be 1200 MB which is more than 1GB. That somewhat restricts program design.
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