From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
Cc: Sean Christopherson <seanjc@google.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH RFC 0/4] KVM: x86: Drastically raise KVM_USER_MEM_SLOTS limit
Date: Wed, 27 Jan 2021 18:55:38 +0100 [thread overview]
Message-ID: <87czxq2rj9.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <40efcfa8-c625-80b5-7ac9-da7839ed6011@oracle.com>
"Maciej S. Szmigiero" <maciej.szmigiero@oracle.com> writes:
> On 15.01.2021 14:18, Vitaly Kuznetsov wrote:
>> TL;DR: any particular reason why KVM_USER_MEM_SLOTS is so low?
>>
>> Longer version:
>>
>> Current KVM_USER_MEM_SLOTS limit (509) can be a limiting factor for some
>> configurations. In particular, when QEMU tries to start a Windows guest
>> with Hyper-V SynIC enabled and e.g. 256 vCPUs the limit is hit as SynIC
>> requires two pages per vCPU and the guest is free to pick any GFN for
>> each of them, this fragments memslots as QEMU wants to have a separate
>> memslot for each of these pages (which are supposed to act as 'overlay'
>> pages).
>>
>> Memory slots are allocated dynamically in KVM when added so the only real
>> limitation is 'id_to_index' array which is 'short'. We don't have any
>> KVM_MEM_SLOTS_NUM/KVM_USER_MEM_SLOTS-sized statically defined arrays.
>>
>> We could've just raised the limit to e.g. '1021' (we have 3 private
>> memslots on x86) and this should be enough for now as KVM_MAX_VCPUS is
>> '288' but AFAIK there are plans to raise this limit as well.
>>
>
> I have a patch series that reworks the whole memslot thing, bringing
> performance improvements across the board.
> Will post it in few days, together with a new mini benchmark set.
I'm about to send a successor of this series. It will be implmenting
Sean's idea to make the maximum number of memslots a per-VM thing (and
also raise the default). Hope it won't interfere with your work!
--
Vitaly
next prev parent reply other threads:[~2021-01-27 17:57 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-15 13:18 [PATCH RFC 0/4] KVM: x86: Drastically raise KVM_USER_MEM_SLOTS limit Vitaly Kuznetsov
2021-01-15 13:18 ` [PATCH RFC 1/4] KVM: x86: Drop redundant KVM_MEM_SLOTS_NUM definition Vitaly Kuznetsov
2021-01-15 16:47 ` Sean Christopherson
2021-01-15 13:18 ` [PATCH RFC 2/4] KVM: mips: Drop KVM_PRIVATE_MEM_SLOTS definition Vitaly Kuznetsov
2021-01-15 13:18 ` [PATCH RFC 3/4] KVM: Define KVM_USER_MEM_SLOTS in arch-neutral include/linux/kvm_host.h Vitaly Kuznetsov
2021-01-15 17:05 ` Sean Christopherson
2021-01-18 9:52 ` Vitaly Kuznetsov
2021-01-19 17:20 ` Sean Christopherson
2021-01-20 11:34 ` Igor Mammedov
2021-01-20 12:02 ` Vitaly Kuznetsov
2021-01-20 17:25 ` Sean Christopherson
2021-01-15 13:18 ` [PATCH RFC 4/4] KVM: x86: Stop limiting KVM_USER_MEM_SLOTS Vitaly Kuznetsov
2021-01-15 16:03 ` [PATCH RFC 0/4] KVM: x86: Drastically raise KVM_USER_MEM_SLOTS limit Sean Christopherson
2021-01-15 16:23 ` Vitaly Kuznetsov
2021-01-15 16:45 ` Sean Christopherson
2021-01-15 18:47 ` Maciej S. Szmigiero
2021-01-27 17:55 ` Vitaly Kuznetsov [this message]
2021-01-27 22:26 ` Maciej S. Szmigiero
2021-01-28 8:47 ` Vitaly Kuznetsov
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