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Re: [RFC PATCH 1/7] Shrink struct vm_page size

From: Sergey Bugaev
Subject: Re: [RFC PATCH 1/7] Shrink struct vm_page size
Date: Mon, 3 Jul 2023 10:23:33 +0300


On Mon, Jul 3, 2023 at 3:02 AM Samuel Thibault <samuel.thibault@gnu.org> wrote:
> I applied this, *but* I see:
>  *      Fields in this structure are locked either by the lock on the
>  *      object that the page belongs to (O) or by the lock on the page
>  *      queues (P).  [Some fields require that both locks be held to
>  *      change that field; holding either lock is sufficient to read.]
> That's not going to work at all with bitfields...

Great point. (But I don't think this patch makes it any worse.)

> We do need to sort this out by separating bitfields that are not
> protected the same way, otherwise it's no wonder if SMP fails oddly.

This is really interesting, because Mach surely was developed for SMP
from the start ("an OS for a parallel computer" was an explicit design
goal, and M in Mach stands for multi-processor), so it must have
worked for them. But indeed I see that they have separated the
bitfields in XNU, so we should too. Hopefully without blowing the
struct size back up.

Any feedback/comments on the rest of the series? Does forward entry
coalescing work for you? Any issues / noticeable performance effects?


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