Re: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush

From: Vitaly Kuznetsov
Date: Thu Jul 13 2017 - 08:46:30 EST

Andy Lutomirski <luto@xxxxxxxxxx> writes:

> On Tue, May 23, 2017 at 5:36 AM, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>> Andy Lutomirski <luto@xxxxxxxxxx> writes:
>>> Also, can you share the benchmark you used for these patches?
>> I didn't do much while writing the patchset, mostly I was running the
>> attached dumb trasher (32 pthreads doing mmap/munmap). On a 16 vCPU
>> Hyper-V 2016 guest I get the following (just re-did the test with
>> 4.12-rc1):
>> Before the patchset:
>> # time ./pthread_mmap ./randfile
>> real 3m33.118s
>> user 0m3.698s
>> sys 3m16.624s
>> After the patchset:
>> # time ./pthread_mmap ./randfile
>> real 2m19.920s
>> user 0m2.662s
>> sys 2m9.948s
>> K. Y.'s guys at Microsoft did additional testing for the patchset on
>> different Hyper-V deployments including Azure, they may share their
>> findings too.
> I ran this benchmark on my big TLB patchset, mainly to make sure I
> didn't regress your test. I seem to have sped it up by 30% or so
> instead. I need to study this a little bit to figure out why to make
> sure that the reason isn't that I'm failing to do flushes I need to
> do.

Got back to this and tested everything on WS2016 Hyper-V guest (24
vCPUs) with my slightly modified benchmark. The numbers are:

1) pre-patch:

real 1m15.775s
user 0m0.850s
sys 1m31.515s

2) your 'x86/pcid' series (PCID feature is not passed to the guest so this
is mainly your lazy tlb optimization):

real 0m55.135s
user 0m1.168s
sys 1m3.810s

3) My 'pv tlb shootdown' patchset on top of your 'x86/pcid' series:

real 0m48.891s
user 0m1.052s
sys 0m52.591s

As far as I understand I need to add
'setup_clear_cpu_cap(X86_FEATURE_PCID)' to my series to make things work
properly if this feature appears in the guest.

Other than that there is an additional room for optimization:
tlb_single_page_flush_ceiling, I'm not sure that with Hyper-V's PV the
default value of 33 is optimal. But the investigation can be done

AFAIU with your TLB preparatory work which got into 4.13 our series
become untangled and can go through different trees. I'll rebase mine
and send it to K. Y. to push through Greg's char-misc tree.

Is there anything blocking your PCID series from going into 4.14? It
seems to big a huge improvement for some workloads.