1. On modern OSes, you probably aren't "taking it away from other processes" until you actually use it. Statically allocated but untouched memory is probably just an entry in a page table somewhere.

2. Speed improvement? No. The improvement is in your ability to reason about memory usage, and about time usage. Dynamic allocations add a very much non-deterministic amount of time to whatever you're doing.

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pastage 39 minutes ago | parent | next [-]

Use mlock as long as it is allocated it is going to be rather deterministic, of course you might be running in a VM on an over commited host. I guess you can "prefault" in a busy loop instead of only on startup, waste memory and cpu!

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Ericson2314 5 hours ago | parent | prev | next [-]

If you use it and stop using it, the OS cannot reclaim the pages, because it doesn't know that you've stopped. At best, it can offload the memory to disk, but this waste disk space, and also time for pointless writes.

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norir 5 hours ago | parent [-]

This is true, whether it matters is context dependent. In an embedded program, this may be irrelevant since your program is the only thing running so there is no resource contention or need to swap. In multi-tenant, you could use arenas in an identical way as single static allocation and release the arena upon completion. I agree that allocating a huge amount of memory for a long running program on a multi-tenant os is a bad idea in general, but it could be ok if for example you are running a single application like a database on the server in which you are back to embedded programming only the embedding is a database on a beefy general purpose computer.

	▲	Ericson2314 3 hours ago \| parent [-]
		Yes it is context dependent, but the parent comment was acting as if it was just better. I wanted to correct that.

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jrmg 3 hours ago | parent | prev | next [-]

In response to (1) - you’re right, but that also implies that the added safety from static allocation when running on a modern OS is just an illusion: the OS may be unable to supply a fresh page from your ‘statically allocated’ memory when you actually write to it and it has to be backed by something real. The real stuff may have run out.

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kibwen 4 hours ago | parent | prev | next [-]

> On modern OSes, you probably aren't "taking it away from other processes" until you actually use it.

But if you're assuming that overcommit is what will save you from wasting memory in this way, then that sabotages the whole idea of using this scheme in order to avoid potential allocation errors.

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jeffreygoesto 4 hours ago | parent | prev [-]

Using this as well in embddded. The whole point is to commit and lock the pages after allocation, to not experience what you correctly describe. You want to have a single checkpoint after which you simply can stop worrying about oom.