From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Greg Kurz <gkurz@linux.vnet.ibm.com>,
Paolo Bonzini <pbonzini@redhat.com>,
james.hogan@imgtec.com, Ingo Molnar <mingo@redhat.com>,
linux-mips@linux-mips.org, kvm <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
qemu-ppc@nongnu.org, Cornelia Huck <cornelia.huck@de.ibm.com>,
Paul Mackerras <paulus@samba.org>,
David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [PATCH v3] KVM: remove buggy vcpu id check on vcpu creation
Date: Fri, 22 Apr 2016 15:07:06 +0200 [thread overview]
Message-ID: <20160422130705.GD7202@potion> (raw)
In-Reply-To: <CANRm+Cwh__btdC4e4t+jYqHsafL6xff6t4eukxT=EmwVLYvrMA@mail.gmail.com>
2016-04-22 09:40+0800, Wanpeng Li:
> 2016-04-21 23:29 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
>> x86 vcpu_id encodes APIC ID and APIC ID encodes CPU topology by
>> reserving blocks of bits for socket/core/thread, so if core or thread
>> count isn't a power of two, then the set of valid APIC IDs is sparse,
>
> ^^^^^^^^^^^^^^^^^^^
> ^^^^^^^
> Is this the root reason why recommand max vCPUs per vm is 160 and the
> KVM_MAX_VCPUS is 255 instead of due to perforamnce concern?
No, the recommended amout of VCPUs is 160 because I didn't bump it after
PLE stopped killing big guests. :/
You can get full 255 VCPU guest with a proper configuration, e.g.
"-smp 255" or "-smp 255,cores=8" and the only problem is scalability,
but I don't know of anything that doesn't scale to that point.
(Scaling up to 2^32 is harder, because you don't want O(N) search, nor
full allocation on smaller guests. Neither is a big problem now.)
next prev parent reply other threads:[~2016-04-22 13:07 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-20 15:44 [PATCH v3] KVM: remove buggy vcpu id check on vcpu creation Greg Kurz
2016-04-20 16:10 ` James Hogan
2016-04-20 16:10 ` James Hogan
2016-04-20 16:48 ` Cornelia Huck
2016-04-21 13:24 ` David Hildenbrand
2016-04-20 17:02 ` Radim Krčmář
2016-04-20 17:09 ` James Hogan
2016-04-20 17:09 ` James Hogan
2016-04-20 17:27 ` Radim Krčmář
2016-04-20 17:53 ` Greg Kurz
2016-04-20 18:31 ` Radim Krčmář
2016-04-20 18:29 ` Radim Krčmář
2016-04-21 11:29 ` Greg Kurz
2016-04-21 11:29 ` Greg Kurz
2016-04-21 12:26 ` Cornelia Huck
2016-04-21 13:05 ` Greg Kurz
2016-04-21 13:22 ` David Hildenbrand
2016-04-21 15:29 ` Radim Krčmář
2016-04-21 15:49 ` Greg Kurz
2016-04-21 16:08 ` Radim Krčmář
2016-04-21 17:18 ` Greg Kurz
2016-04-21 17:39 ` Radim Krčmář
2016-04-21 18:08 ` Greg Kurz
2016-04-22 1:40 ` Wanpeng Li
2016-04-22 13:07 ` Radim Krčmář [this message]
2016-04-23 22:54 ` Wanpeng Li
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