linux-kernel - Re: [PATCH 1/1] mm: Export split

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Date:	Mon, 04 Mar 2013 09:05:58 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	KY Srinivasan <kys@...rosoft.com>
CC:	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 1/1] mm: Export split_page().

On 03/03/2013 06:36 PM, KY Srinivasan wrote:
>> I guess the most obvious question about exporting this symbol is, "Why
>> doesn't any of the other hypervisor balloon drivers need this?  What is
>> so special about hyper-v?"
> 
> The balloon protocol that Hyper-V has specified is designed around the ability to
> move 2M pages. While the protocol can handle 4k allocations, it is going to be very chatty
> with 4K allocations.

What does "very chatty" mean?  Do you think that there will be a
noticeable performance difference ballooning 2M pages vs 4k?

> Furthermore, the Memory Balancer on the host is also designed to work
> best with memory moving around in 2M chunks. While I have not seen the code on the Windows
> host that does this memory balancing, looking at how Windows guests behave in this environment,
> (relative to Linux) I have to assume that the 2M allocations that Windows guests do are a big part of
> the difference we see.

You've been talking about differences.  Could you elaborate on what the
differences in behavior are that you are trying to rectify here?

>> Or can those other drivers also need/use it as well, and they were just
>> too chicken to be asking for the export?  :)
> 
> The 2M balloon allocations would make sense if the host is designed accordingly.

How does the guest decide which size pages to allocate?  It seems like a
relatively bad idea to be inflating the balloon with 2M pages from the
guest in the case where the guest is under memory pressure _and_
fragmented.

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