Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

[David Hildenbrand]

On 19.02.19 15:40, Michael S. Tsirkin wrote:
> On Tue, Feb 19, 2019 at 09:06:01AM +0100, David Hildenbrand wrote:
>> On 19.02.19 00:47, Alexander Duyck wrote:
>>> On Mon, Feb 18, 2019 at 9:42 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>
>>>> On 18.02.19 18:31, Alexander Duyck wrote:
>>>>> On Mon, Feb 18, 2019 at 8:59 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>>>
>>>>>> On 18.02.19 17:49, Michael S. Tsirkin wrote:
>>>>>>> On Sat, Feb 16, 2019 at 10:40:15AM +0100, David Hildenbrand wrote:
>>>>>>>> It would be worth a try. My feeling is that a synchronous report after
>>>>>>>> e.g. 512 frees should be acceptable, as it seems to be acceptable on
>>>>>>>> s390x. (basically always enabled, nobody complains).
>>>>>>>
>>>>>>> What slips under the radar on an arch like s390 might
>>>>>>> raise issues for a popular arch like x86. My fear would be
>>>>>>> if it's only a problem e.g. for realtime. Then you get
>>>>>>> a condition that's very hard to trigger and affects
>>>>>>> worst case latencies.
>>>>>>
>>>>>> Realtime should never use free page hinting. Just like it should never
>>>>>> use ballooning. Just like it should pin all pages in the hypervisor.
>>>>>>
>>>>>>>
>>>>>>> But really what business has something that is supposedly
>>>>>>> an optimization blocking a VCPU? We are just freeing up
>>>>>>> lots of memory why is it a good idea to slow that
>>>>>>> process down?
>>>>>>
>>>>>> I first want to know that it is a problem before we declare it a
>>>>>> problem. I provided an example (s390x) where it does not seem to be a
>>>>>> problem. One hypercall ~every 512 frees. As simple as it can get.
>>>>>>
>>>>>> No trying to deny that it could be a problem on x86, but then I assume
>>>>>> it is only a problem in specific setups.
>>>>>>
>>>>>> I would much rather prefer a simple solution that can eventually be
>>>>>> disabled in selected setup than a complicated solution that tries to fit
>>>>>> all possible setups. Realtime is one of the examples where such stuff is
>>>>>> to be disabled either way.
>>>>>>
>>>>>> Optimization of space comes with a price (here: execution time).
>>>>>
>>>>> One thing to keep in mind though is that if you are already having to
>>>>> pull pages in and out of swap on the host in order be able to provide
>>>>> enough memory for the guests the free page hinting should be a
>>>>> significant win in terms of performance.
>>>>
>>>> Indeed. And also we are in a virtualized environment already, we can
>>>> have any kind of sudden hickups. (again, realtime has special
>>>> requirements on the setup)
>>>>
>>>> Side note: I like your approach because it is simple. I don't like your
>>>> approach because it cannot deal with fragmented memory. And that can
>>>> happen easily.
>>>>
>>>> The idea I described here can be similarly be an extension of your
>>>> approach, merging in a "batched reporting" Nitesh proposed, so we can
>>>> report on something < MAX_ORDER, similar to s390x. In the end it boils
>>>> down to reporting via hypercall vs. reporting via virtio. The main point
>>>> is that it is synchronous and batched. (and that we properly take care
>>>> of the race between host freeing and guest allocation)
>>>
>>> I'd say the discussion is even simpler then that. My concern is more
>>> synchronous versus asynchronous. I honestly think the cost for a
>>> synchronous call is being overblown and we are likely to see the fault
>>> and zeroing of pages cost more than the hypercall or virtio
>>> transaction itself.
>>
>> The overhead of page faults and zeroing should be mitigated by
>> MADV_FREE, as Andrea correctly stated (thanks!). Then, the call overhead
>> (context switch) becomes relevant.
>>
>> We have various discussions now :) And I think they are related.
>>
>> synchronous versus asynchronous
>> batched vs. non-batched
>> MAX_ORDER - 1 vs. other/none magic number
>>
>> 1. synchronous call without batching on every kfree is bad. The
>> interface is fixed to big magic numbers, otherwise we end up having a
>> hypercall on every kfree. This is your approach.
>>
>> 2. asynchronous calls without batching would most probably have similar
>> problems with small granularities as we had when ballooning without
>> batching. Just overhead we can avoid.
>>
>> 3. synchronous and batched is what s390x does. It can deal with page
>> granularity. It is what I initially described in this sub-thread.
>>
>> 4. asynchronous and batched. This is the other approach we discussed
>> yesterday. If we can get it implemented, I would be interested in
>> performance numbers.
>>
>> As far as I understood, Michael seems to favor something like 4 (and I
>> assume eventually 2 if it is similarly fast). I am a friend of either 3
>> or 4.
> 
> Well Linus said big granularity is important for linux MM
> and not to bother with hinting small sizes.
> 

For some reason I tend to challenge also the opinions of people way
smarter than me ;) Only the numbers can tell the true story later.

> Alex said cost of a hypercall is drawfed by a pagefault
> after alloc. I would be curious whether async pagefault
> can help things somehow though.

Indeed.

> 
>>>
>>> Also one reason why I am not a fan of working with anything less than
>>> PMD order is because there have been issues in the past with false
>>> memory leaks being created when hints were provided on THP pages that
>>> essentially fragmented them. I guess hugepaged went through and
>>> started trying to reassemble the huge pages and as a result there have
>>> been apps that ended up consuming more memory than they would have
>>> otherwise since they were using fragments of THP pages after doing an
>>> MADV_DONTNEED on sections of the page.
>>
>> I understand your concerns, but we should not let bugs in the hypervisor
>> dictate the design. Bugs are there to be fixed. Interesting read,
>> though, thanks!
> 
> Right but if we break up a huge page we are then creating
> more work for hypervisor to reassemble it.

Yes, but the hypervisor can decide what to do. E.g. on s390x there are
no THP, so nothing to break up.

It is all very complicated :)

-- 

Thanks,

David / dhildenb