linux-kernel - Re: [Xen-devel] [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux

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Message-ID: <55F819A1.8070609@citrix.com>
Date:	Tue, 15 Sep 2015 14:14:09 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Arnd Bergmann <arnd@...db.de>,
	<linux-arm-kernel@...ts.infradead.org>
CC:	<wei.liu2@...rix.com>, <ian.campbell@...rix.com>,
	<stefano.stabellini@...citrix.com>, <linux-kernel@...r.kernel.org>,
	"Julien Grall" <julien.grall@...rix.com>,
	<david.vrabel@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<boris.ostrovsky@...cle.com>,
	Roger Pau Monné <roger.pau@...rix.com>
Subject: Re: [Xen-devel] [PATCH v4 00/20] xen/arm64: Add support for 64KB
 page in Linux

On 14/09/15 12:32, Arnd Bergmann wrote:
> On Monday 14 September 2015 13:04:59 Roger Pau Monné wrote:
>>> TBH, I'm expecting a small impact to the performance. It would be hard
>>> to get the exactly the same performance as today if we keep the helpers
>>> to avoid the backend dealing himself with the splitting and page
>>> granularity.
>>>
>>> Although, if the performance impact is not acceptable, it may be
>>> possible to optimize gnttab_foreach_grant_in_range by moving the
>>> function inline. The current way to the loop is the fastest I've found
>>> (I've wrote a small program to test different way) and we will need it
>>> when different of size will be supported.
>>
>> I don't expect the performance to drop massively with this patches
>> applied, but it would be good to al least have an idea of the impact.
> 
> Note that using 64kb pages in Linux tends to destroy performance
> in Linux in any case, as the memory consumption for most workloads
> explodes. In a virtualized environment you already tend to be
> memory constrained, so any measurement should take that into account
> and put the extra overhead into perspective to the massive overhead
> of running 64kb pages when RAM is tight.

If this is the case, why are some distros using 64 KiB pages then?

David
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