> -----Original Message----- > From: Dave Hansen [mailto:d...@linux.vnet.ibm.com] > Sent: Monday, March 04, 2013 12:06 PM > To: KY Srinivasan > Cc: Greg KH; linux-kernel@vger.kernel.org; de...@linuxdriverproject.org; > o...@aepfle.de; a...@canonical.com; a...@firstfloor.org; akpm@linux- > foundation.org; linux...@kvack.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?
The balloon protocol that Hyper-V host specified allows you to specify page ranges - start_pfn: num_pfn. With 2M pages the number of messages that need to be exchanges is significantly fewer than with 4K page allocations. > > > 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? As I look at how smoothly memory is balanced on Windows guests with changing load conditions in the guest relative to what I see with Linux, I see Linux taking more time to reach the steady state during a balancing operation. I will experiment with 2M allocations and report if this issue is addressed. > > >> 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. I want to start with 2M allocations and if they fail, fall back onto lower order allocations. As I said, the host can support 4K allocations and that will be the final fallback position (that is what I have currently implemented). If the guest memory is fragmented, then obviously we will go in for lower order allocations. Regards, K. Y -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
