linux-kernel - Re: [PATCH] staging: lustre: llite: Use kzalloc and rewrite null tests

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Date:	Fri, 19 Sep 2014 13:27:49 +0000
From:	"Drokin, Oleg" <oleg.drokin@...el.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
CC:	Julia Lawall <Julia.Lawall@...6.fr>,
	"<devel@...verdev.osuosl.org>" <devel@...verdev.osuosl.org>,
	"Dilger, Andreas" <andreas.dilger@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"<kernel-janitors@...r.kernel.org>" <kernel-janitors@...r.kernel.org>,
	"<linux-kernel@...r.kernel.org>" <linux-kernel@...r.kernel.org>,
	"<HPDD-discuss@...1.01.org>" <HPDD-discuss@...1.01.org>
Subject: Re: [PATCH] staging: lustre: llite: Use kzalloc and rewrite null
 tests


On Sep 19, 2014, at 5:11 AM, Dan Carpenter wrote:

> On Fri, Sep 19, 2014 at 02:57:03AM +0000, Drokin, Oleg wrote:
>> 4. Sometimes we need large allocations. general kmalloc is less
>> reliable as system lives on and memory fragmentation worsens. So we
>> have this "allocations over 2-4 pages get switched to vmalloc" logic,
>> if there's a way to do that automatically - that would be great.
> 
> Julia's patch only changes OBD_ALLOC() functions and those are always
> kmalloc so that's not an issue here.

That's true. Though it would be strange to convert just a subset of those
allocating macros, and the solution for the OBD_ALLOC_LARGE would
also be needed.
> 
> The OBD_ALLOC_LARGE() macro is vmalloc() or kmalloc() if the size is
> small enough.  We don't really want to choose between kmalloc and
> vmalloc automatically.  My instinct is that we should change all the
> OBD_ALLOC_LARGE() to vmalloc() and trust it to allocate them in the most
> sane way possible.  But I haven't really looked very closely.

We found that vmalloc (at least with larger allocs, I did not look into details)
has quite a big penalty with some internal lock that creates big contention at
production loads.
So of course the desire is there to reduce our allocations when we can.
(additionally on 32 bit arches there's always this issue of vmalloc region
possibly not being large enough to allocate everything we might want whenever
we see a way for that).

Thanks.

Bye,
    Oleg--
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