linux-kernel - Re: [PATCH v2] MIPS64: Support of at least 48 bits of SEGBITS

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Date:	Fri, 15 May 2015 15:39:54 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	Ralf Baechle <ralf@...ux-mips.org>
CC:	<aleksey.makarov@...iga.com>, <james.hogan@...tec.com>,
	<paul.burton@...tec.com>, <david.daney@...ium.com>,
	<peterz@...radead.org>, <linux-mips@...ux-mips.org>,
	<linux-kernel@...r.kernel.org>, <davidlohr@...com>,
	<kirill@...temov.name>, <akpm@...ux-foundation.org>,
	<mingo@...nel.org>
Subject: Re: [PATCH v2] MIPS64: Support of at least 48 bits of SEGBITS

On 05/15/2015 02:53 PM, Ralf Baechle wrote:
> On Thu, May 14, 2015 at 06:34:43PM -0700, Leonid Yegoshin wrote:
>
> The order 1 allocation for the PGD are concerning me a little.  On a
> system under even moderate memory pressure that might become a bit of
> a reliability or performance issue.
>
> With 4kB pages we already need order 1 or even 2 allocations for the
> allocation of the stack and some folks have reported that to be an issue
> so we may have to start using the PUD for very large VA spaces.
>
>    Ralf

I don't think it is an issue here - people, who wants to exercise 256 
TERABAIT of memory PER PROCESS may even doesn't note that they have PGD 
= 2 pages. It is definitely not for systems with 4GB physmemory.

I also recommend for low memory to look into CONFIG_COMPACTION, it may 
be a great help for them here, look into mm/vmscan.c, 
in_reclaim_compaction().

Besides that, I defined this feature for 16KB and 64KB pages only, not 
for 4KB.


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