Re: [PATCH 4/4] KVM: MMU: Make mmu_shrink() scan nr_to_scan shadow pages

[Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>]

(2011/12/19 18:26), Avi Kivity wrote:
> On 12/19/2011 11:22 AM, Takuya Yoshikawa wrote:
>>> Yes, it's very conservative.  But on the other hand the shrinker is
>>> tuned for dcache and icache, where there are usually tons of useless
>>> objects.  If we have to free something, I'd rather free them instead of
>>> mmu pages which tend to get recreated soon.
>>>
>>
>>
>> OK, to satisfy the requirements, I will do:
>>
>>      1. find the guest with the highest (shadow pages / memory) ratio
>
> How do you propose to do that efficiently?  We may have hundreds of
> guests, or even more, on one host.  Each guest access will involve
> bouncing a few cache lines.

IMO, The goal should be restricted to emergencies.

So possible solution may be:
	- we set the tuning parameters as conservative as possible
	- pick up a guest with relatively high ratio
	  (I have to think more how to achieve this)
	- move the vm_list head for fairness

In an emergency, we should not mind performance penalty so much.

>
>>      2. just zap one page from that guest, keeping the current
>> conservative rate
>>
>> I will update the patch.
>
> I think the current rate is too conservative.  No idea what a good one
> is, I don't have a feeling as to the relation between shrinker callbacks
> and memory pressure.
>

When I tried to see what the current code is doing, frankly speaking,
I thought mmu_shrink() was not tested enough from the beginning.

I read the shrinker code as far as possible and realized the combination of
(seeks=10*default, batch=128) is not reasonable; the high seeks means the
shrinker rarely calculate higher value than 128, and mmu_shrink() cannot
be called in normal life.

How about setting the batch a bit lower, keeping seeks as is?

But there is not a perfect value because how often mmu_shrink() can be called
will change if the admin change the sysctl_vfs_cache_pressure tuning parameter
for dcache and icache, IIUC.

And tdp and shadow paging differ much.

	Takuya