linux-kernel - Re: increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty

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Message-ID: <5244AA5C.8060100@hurleysoftware.com>
Date:	Thu, 26 Sep 2013 17:42:52 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Greg KH <gregkh@...uxfoundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org,
	Tejun Heo <tj@...nel.org>
Subject: Re: increased vmap_area_lock contentions on "n_tty: Move buffers
 into n_tty_data"

On 09/26/2013 02:05 PM, Andrew Morton wrote:
> On Thu, 26 Sep 2013 13:35:32 -0400 Peter Hurley <peter@...leysoftware.com> wrote:
>
>> The issue with a single large kmalloc is that it may fail where
>> 3 separate, page-or-less kmallocs would not have.
>
> Or vmalloc fails first, because of internal fragmentation of the vmap
> arena.  This problem plus vmalloc's slowness are the reasons why
> vmalloc should be avoided.

Ok, no vmalloc.

> A tremendous number of places in the kernel perform higher-order
> allocations nowadays.  The page allocator works damn hard to service
> them and I expect that switching to kmalloc here will be OK.

I've had order-4 allocation failures before on 10Gb. In fact, the
nouveau driver switched to vmalloc for that very reason (commit
d005f51eb93d71cd40ebd11dd377453fa8c8a42a, drm/nouveau: use vmalloc
for pgt allocation).

Regards,
Peter Hurley
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