linux-kernel - Re: Trying to make use of hotplug memory for xen balloon driver

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Message-Id: <20080327151115.be9f325d.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 27 Mar 2008 15:11:15 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Yasunori Goto <y-goto@...fujitsu.com>,
	Christoph Lameter <clameter@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: Trying to make use of hotplug memory for xen balloon driver

On Wed, 26 Mar 2008 22:57:57 -0700
Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Ah, I see what it is.  I wasn't trying to add enough memory.  It adds in 
> units of SECTION_SIZE_BITS, which is 2^30 on 32-bit PAE.  When I 
> increase the initial balloon extension to PAGES_PER_SECTION pages, I 
> make some more progress:
> 
> xen_balloon: Initialising balloon driver.
> trying to reserve 262144 pages (1073741824 bytes) for balloon
> bootmem alloc of 147456 bytes failed!
> Kernel panic - not syncing: Out of memory
> Pid: 1, comm: swapper Not tainted 2.6.25-rc7-x86-latest.git-dirty #361
>  [<c01299dc>] panic+0x49/0x102
>  [<c0647c3c>] __alloc_bootmem+0x24/0x29
>  [<c0647c6d>] __alloc_bootmem_node+0x2c/0x34
>  [<c044bd97>] zone_wait_table_init+0x45/0x95
>  [<c0467258>] init_currently_empty_zone+0x1d/0xaa
>  [<c01738ea>] __add_pages+0x88/0xdb
>  [<c011c1a5>] arch_add_memory+0x25/0x2b
>  [<c01737a9>] add_memory_resource+0x2f/0x36
>  [<c064e487>] balloon_init+0x1b8/0x2b9
>  [<c0635495>] kernel_init+0x137/0x292
>  [<c063535e>] ? kernel_init+0x0/0x292
>  [<c063535e>] ? kernel_init+0x0/0x292
>  [<c0108b67>] kernel_thread_helper+0x7/0x10
>  =======================
> 
> 
> What's the rationale for setting SECTION_SIZE_BITS to 30?  Seems like a 
> fairly large chunk.
> 
At first, I believe usual DIMM size is bigger than SECTION_SIZE_BITS. This is
designed for hardware-based hotplug.

If you want to use memory-hotplug for virtualized enviroment, it's good to make
this to be smaller chunk. Powerpc/IBM lpar uses 16MB chunk.

It's a trade-off between section mainainance cost v.s. size of plugged memory.
please find the best.

Thanks,
-Kame






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