Messages in this thread |  | | Date | Mon, 27 Jul 2020 18:12:34 +0100 | From | Marc Zyngier <> | Subject | Re: [RESEND RFC PATCH v1] arm64: kvm: flush tlbs by range in unmap_stage2_range function |
| |
Zhenyu,
On 2020-07-27 15:51, Zhenyu Ye wrote: > Hi Marc, > > On 2020/7/26 1:40, Marc Zyngier wrote: >> On 2020-07-24 14:43, Zhenyu Ye wrote: >>> Now in unmap_stage2_range(), we flush tlbs one by one just after the >>> corresponding pages cleared. However, this may cause some >>> performance >>> problems when the unmap range is very large (such as when the vm >>> migration rollback, this may cause vm downtime too loog). >> >> You keep resending this patch, but you don't give any numbers >> that would back your assertion. > > I have tested the downtime of vm migration rollback on arm64, and found > the downtime could even take up to 7s. Then I traced the cost of > unmap_stage2_range() and found it could take a maximum of 1.2s. The > vm configuration is as follows (with high memory pressure, the dirty > rate is about 500MB/s): > > <memory unit='GiB'>192</memory> > <vcpu placement='static'>48</vcpu> > <memoryBacking> > <hugepages> > <page size='1' unit='GiB' nodeset='0'/> > </hugepages> > </memoryBacking>
This means nothing to me, I'm afraid.
> > After this patch applied, the cost of unmap_stage2_range() can reduce > to > 16ms, and VM downtime can be less than 1s. > > The following figure shows a clear comparison: > > | vm downtime | cost of unmap_stage2_range() > --------------+--------------+---------------------------------- > before change | 7s | 1200 ms > after change | 1s | 16 ms > --------------+--------------+----------------------------------
I don't see how you turn a 1.184s reduction into a 6s gain. Surely there is more to it than what you posted.
>>> + >>> + if ((end - start) >= 512 << (PAGE_SHIFT - 12)) { >>> + __tlbi(vmalls12e1is); >> >> And what is this magic value based on? You don't even mention in the >> commit log that you are taking this shortcut. >> > > > If the page num is bigger than 512, flush all tlbs of this vm to avoid > soft lock-ups on large TLB flushing ranges. Just like what the > flush_tlb_range() does.
I'm not sure this is applicable here, and it doesn't mean this is as good on other systems.
Thanks,
M. -- Jazz is not dead. It just smells funny...
|  |