Re: [PATCH] ipc: optimize semget/shmget/msgget for lots of keys

[Davidlohr Bueso]

On Mon, 31 Jul 2017, Guillaume Knispel wrote:

> ipc_findkey() scanned all objects to look for the wanted key. This is
> slow when using a high number of keys, for example on an i5 laptop the
> following loop took 17 s, with last semget calls taking ~1 ms each.

I would argue that this is not the common case.

>    for (int i = 0, k=0x424242; i < 31900; ++i)
>        semget(k++, 1, IPC_CREAT | 0600);
>
> This change adds an rhashtable of kern_ipc_perm objects in ipc_ids, so
> that one lookup cease to be O(n). The above loop now takes ~10 ms. Each
> lookup-only semget() call can take between ~120 ns and a few µs. Rarely,
> some creations can take a few dozen of µs.

Could you please provide numbers for smaller amounts of keys?

Thanks,
Davidlohr