Re: [RFC][Patch v9 2/6] KVM: Enables the kernel to isolate guest free pages

[Nitesh Narayan Lal]

On 3/8/19 2:25 PM, Alexander Duyck wrote:
> On Fri, Mar 8, 2019 at 11:10 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:
>>
>> On 3/8/19 1:06 PM, Alexander Duyck wrote:
>>> On Thu, Mar 7, 2019 at 6:32 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>>>> On Thu, Mar 07, 2019 at 02:35:53PM -0800, Alexander Duyck wrote:
>>>>> The only other thing I still want to try and see if I can do is to add
>>>>> a jiffies value to the page private data in the case of the buddy
>>>>> pages.
>>>> Actually there's one extra thing I think we should do, and that is make
>>>> sure we do not leave less than X% off the free memory at a time.
>>>> This way chances of triggering an OOM are lower.
>>> If nothing else we could probably look at doing a watermark of some
>>> sort so we have to have X amount of memory free but not hinted before
>>> we will start providing the hints. It would just be a matter of
>>> tracking how much memory we have hinted on versus the amount of memory
>>> that has been pulled from that pool.
>> This is to avoid false OOM in the guest?
> Partially, though it would still be possible. Basically it would just
> be a way of determining when we have hinted "enough". Basically it
> doesn't do us much good to be hinting on free memory if the guest is
> already constrained and just going to reallocate the memory shortly
> after we hinted on it. The idea is with a watermark we can avoid
> hinting until we start having pages that are actually going to stay
> free for a while.
>
>>>  It is another reason why we
>>> probably want a bit in the buddy pages somewhere to indicate if a page
>>> has been hinted or not as we can then use that to determine if we have
>>> to account for it in the statistics.
>> The one benefit which I can see of having an explicit bit is that it
>> will help us to have a single hook away from the hot path within buddy
>> merging code (just like your arch_merge_page) and still avoid duplicate
>> hints while releasing pages.
>>
>> I still have to check PG_idle and PG_young which you mentioned but I
>> don't think we can reuse any existing bits.
> Those are bits that are already there for 64b. I think those exist in
> the page extension for 32b systems. If I am not mistaken they are only
> used in VMA mapped memory. What I was getting at is that those are the
> bits we could think about reusing.
>
>> If we really want to have something like a watermark, then can't we use
>> zone->free_pages before isolating to see how many free pages are there
>> and put a threshold on it? (__isolate_free_page() does a similar thing
>> but it does that on per request basis).
> Right. That is only part of it though since that tells you how many
> free pages are there. But how many of those free pages are hinted?
> That is the part we would need to track separately and then then
> compare to free_pages to determine if we need to start hinting on more
> memory or not.
Only pages which are isolated will be hinted, and once a page is
isolated it will not be counted in the zone free pages.
Feel free to correct me if I am wrong.
If I am understanding it correctly you only want to hint the idle pages,
is that right?
>
>>>>> With that we could track the age of the page so it becomes
>>>>> easier to only target pages that are truly going cold rather than
>>>>> trying to grab pages that were added to the freelist recently.
>>>> I like that but I have a vague memory of discussing this with Rik van
>>>> Riel and him saying it's actually better to take away recently used
>>>> ones. Can't see why would that be but maybe I remember wrong. Rik - am I
>>>> just confused?
>>> It is probably to cut down on the need for disk writes in the case of
>>> swap. If that is the case it ends up being a trade off.
>>>
>>> The sooner we hint the less likely it is that we will need to write a
>>> given page to disk. However the sooner we hint, the more likely it is
>>> we will need to trigger a page fault and pull back in a zero page to
>>> populate the last page we were working on. The sweet spot will be that
>>> period of time that is somewhere in between so we don't trigger
>>> unnecessary page faults and we don't need to perform additional swap
>>> reads/writes.
>> --
>> Regards
>> Nitesh
>>
-- 
Regards
Nitesh

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