Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

Nick Piggin
2005-11-02T09:35:50Z

Gerrit Huizenga wrote:
> On Wed, 02 Nov 2005 19:50:15 +1100, Nick Piggin wrote:

>>Isn't the solution for your hypervisor problem to dish out pages of
>>the same size that are used by the virtual machines. Doesn't this
>>provide you with a nice, 100% solution that doesn't add complexity
>>where it isn't needed?
>
>
> So do you see the problem with fragementation if the hypervisor is
> handing out, say, 1 MB pages? Or, more likely, something like 64 MB
> pages? What are the chances that an entire 64 MB page can be freed
> on a large system that has been up a while?
>

I see the problem, but if you want to be able to shrink memory to a
given size, then you must either introduce a hard limit somewhere, or
have the hypervisor hand out guest sized pages. Use zones, or Xen?

> And, if you create zones, you run into all of the zone rebalancing
> problems of ZONE_DMA, ZONE_NORMAL, ZONE_HIGHMEM. In that case, on
> any long running system, ZONE_HOTPLUGGABLE has been overwhelmed with
> random allocations, making almost none of it available.
>

If there are zone rebalancing problems[*], then it would be great to
have more users of zones because then they will be more likely to get
fixed.

[*] and there are, sadly enough - see the recent patches I posted to
lkml for example. But I'm fairly confident that once the particularly
silly ones have been fixed, zone balancing will no longer be a
derogatory term as has been thrown around (maybe rightly) in this
thread!

--
SUSE Labs, Novell Inc.

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