Message-ID: <555675BA.9000700@imgtec.com>
Date: Fri, 15 May 2015 15:39:54 -0700
From: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
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To: Ralf Baechle <ralf@linux-mips.org>
CC: <aleksey.makarov@auriga.com>, <james.hogan@imgtec.com>,
        <paul.burton@imgtec.com>, <david.daney@cavium.com>,
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        <linux-kernel@vger.kernel.org>, <davidlohr@hp.com>,
        <kirill@shutemov.name>, <akpm@linux-foundation.org>,
        <mingo@kernel.org>
Subject: Re: [PATCH v2] MIPS64: Support of at least 48 bits of SEGBITS
References: <20150515013351.7450.12130.stgit@ubuntu-yegoshin> <20150515215320.GI2322@linux-mips.org>
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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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