Re: [for-4.9] Re: HVM guest performance regression

[Juergen Gross <jgross@suse.com>]

On 30/05/17 12:43, Jan Beulich wrote:
>>>> On 30.05.17 at 12:33, <jgross@suse.com> wrote:
>> On 30/05/17 09:24, Jan Beulich wrote:
>>>>>> On 29.05.17 at 21:05, <jgross@suse.com> wrote:
>>>> Creating the domains with
>>>>
>>>> xl -vvv create ...
>>>>
>>>> showed the numbers of superpages and normal pages allocated for the
>>>> domain.
>>>>
>>>> The following allocation pattern resulted in a slow domain:
>>>>
>>>> xc: detail: PHYSICAL MEMORY ALLOCATION:
>>>> xc: detail:   4KB PAGES: 0x0000000000000600
>>>> xc: detail:   2MB PAGES: 0x00000000000003f9
>>>> xc: detail:   1GB PAGES: 0x0000000000000000
>>>>
>>>> And this one was fast:
>>>>
>>>> xc: detail: PHYSICAL MEMORY ALLOCATION:
>>>> xc: detail:   4KB PAGES: 0x0000000000000400
>>>> xc: detail:   2MB PAGES: 0x00000000000003fa
>>>> xc: detail:   1GB PAGES: 0x0000000000000000
>>>>
>>>> I ballooned dom0 down in small steps to be able to create those
>>>> test cases.
>>>>
>>>> I believe the main reason is that some data needed by the benchmark
>>>> is located near the end of domain memory resulting in a rather high
>>>> TLB miss rate in case of not all (or nearly all) memory available in
>>>> form of 2MB pages.
>>>
>>> Did you double check this by creating some other (persistent)
>>> process prior to running your benchmark? I find it rather
>>> unlikely that you would consistently see space from the top of
>>> guest RAM allocated to your test, unless it consumes all RAM
>>> that's available at the time it runs (but then I'd consider it
>>> quite likely for overhead of using the few smaller pages to be
>>> mostly hidden in the noise).
>>>
>>> Or are you suspecting some crucial kernel structures to live
>>> there?
>>
>> Yes, I do. When onlining memory at boot time the kernel is using the new
>> memory chunk to add the page structures and if needed new kernel page
>> tables. It is normally allocating that memory from the end of the new
>> chunk.
> 
> The page tables are 4k allocations, sure. But the page structures
> surely would be allocated with higher granularity?

I'm really not sure. It might depend on the memory model (sparse,
sparse vmemmap, flat).

>>>>>> What makes the whole problem even more mysterious is that the
>>>>>> regression was detected first with SLE12 SP3 (guest and dom0, Xen 4.9
>>>>>> and Linux 4.4) against older systems (guest and dom0). While trying
>>>>>> to find out whether the guest or the Xen version are the culprit I
>>>>>> found that the old guest (based on kernel 3.12) showed the mentioned
>>>>>> performance drop with above commit. The new guest (based on kernel
>>>>>> 4.4) shows the same bad performance regardless of the Xen version or
>>>>>> amount of free memory. I haven't found the Linux kernel commit yet
>>>>>> being responsible for that performance drop.
>>>>
>>>> And this might be result of a different memory usage of more recent
>>>> kernels: I suspect the critical data is now at the very end of the
>>>> domain's memory. As there are always some pages allocated in 4kB
>>>> chunks the last pages of the domain will never be part of a 2MB page.
>>>
>>> But if the OS allocated large pages internally for relevant data
>>> structures, those obviously won't come from that necessarily 4k-
>>> mapped tail range.
>>
>> Sure? I think the kernel is using 1GB pages if possible for direct
>> kernel mappings of the physical memory. It doesn't care for the last
>> page mapping some space not populated.
> 
> Are you sure? I would very much hope for Linux to not establish
> mappings to addresses where no memory (and no MMIO) resides.
> But I can't tell for sure for recent Linux versions; I do know in the
> old days they were quite careful there.

Looking at phys_pud_init() they are happily using 1GB pages until they
have all memory mapped.


Juergen

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