Date: Mon, 6 Jan 2003 13:00:58 -0500 From: Jake Burkholder <jake@locore.ca> To: Bruce Evans <bde@zeta.org.au> Cc: arch@FreeBSD.ORG Subject: Re: uarea/kstack/pcb/swapout sillyness Message-ID: <20030106130058.B96446@locore.ca> In-Reply-To: <20030106220745.Q2670-100000@gamplex.bde.org>; from bde@zeta.org.au on Mon, Jan 06, 2003 at 10:42:50PM %2B1100 References: <20030106220745.Q2670-100000@gamplex.bde.org>
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Apparently, On Mon, Jan 06, 2003 at 10:42:50PM +1100,
Bruce Evans said words to the effect of;
> Kernel stack pages don't seem to be handled by vm_proc_swap{in,out}.
> Only the upages are swapped. This makes having a user area separate
> from the proc struct even less useful than it used to be. We have
> considerable complications to support the dubious feature of swapping
> the user area. Swapping it out saves a whole 1 page per process
> (UAREA_PAGE is 1 for all arches). OTOH, we don't bother swapping out
> the much larger kernel stacks. There are 4 pages per stack on some
> arches and there may be multiple stacks per process (1 per thread).
>
> Having a separate user area actually wastes memory in the usual (?)
> case where the user area is not swapped. The user area now contains
> only struct sigacts, which has size 2688, and struct pstats, which
> has size 224, so 4096 - 2688 - 224 = 1184 bytes per process is wasted
> for the struct (not counting costs for indirections).
The kernel stack pages are still handled in MD code, due to the fact
that ia64 uses contigmalloc to allocate them, but they are/will still
be swapped. pmap_swapout_thread is called by swapout() on each thread
in a process after swapping out the uarea.
>
> There seems to be even less point to keeping the pcb separate from
> the thread struct. It saves space only by stealing stack space,
> and costs indirections. Some PS_INMEM checks are now bogus because
> the pcb is not in the user area so it doesn't get swapped.
>
> I use "options NO_SWAPPING" to prevent swapping of upages, and would
> like to make this the default and later remove its code and merge the
> structs. Swapping of upages seems to be least useful on large systems
> where it would save the most memory, since such systems might actually
> want to spend time running the processes instead of swapping the them
> see the long message for the commit that introduced NO_SWAPPING in 1996).
> With today's larger memories and bloated processes and kernel stacks,
> I think the space/time savings (positive or negative) don't matter and
> we just have useless complications to swap upages.
Agree. The stuff in the uarea should be allocated normally and hung
off of struct proc.
Wether or not swapping kernel stacks is still useful I don't really know.
Jake
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