From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Date: Fri, 04 Sep 2015 10:07:39 +0000 Subject: Re: [Qemu-ppc] KVM memory slots limit on powerpc Message-Id: <55E96D6B.2070201@de.ibm.com> List-Id: References: <55E965FC.8080909@redhat.com> <55E96B87.8020900@de.ibm.com> <55E96CB9.4070100@suse.de> In-Reply-To: <55E96CB9.4070100@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Alexander Graf , Thomas Huth , kvm-ppc@vger.kernel.org, qemu-ppc@nongnu.org, kvm@vger.kernel.org Cc: David Gibson , Alex Williamson , Bharata B Rao Am 04.09.2015 um 12:04 schrieb Alexander Graf: > > > On 04.09.15 11:59, Christian Borntraeger wrote: >> Am 04.09.2015 um 11:35 schrieb Thomas Huth: >>> >>> Hi all, >>> >>> now that we get memory hotplugging for the spapr machine on qemu-ppc, >>> too, it seems like we easily can hit the amount of KVM-internal memory >>> slots now ("#define KVM_USER_MEM_SLOTS 32" in >>> arch/powerpc/include/asm/kvm_host.h). For example, start >>> qemu-system-ppc64 with a couple of "-device secondary-vga" and "-m >>> 4G,slots2,maxmem@G" and then try to hot-plug all 32 DIMMs ... and >>> you'll see that it aborts way earlier already. >>> >>> The x86 code already increased the amount of KVM_USER_MEM_SLOTS to 509 >>> already (+3 internal slots = 512) ... maybe we should now increase the >>> amount of slots on powerpc, too? Since we don't use internal slots on >>> POWER, would 512 be a good value? Or would less be sufficient, too? >> >> When you are at it, the s390 value should also be increased I guess. > > That constant defines the array size for the memslot array in struct kvm > which in turn again gets allocated by kzalloc, so it's pinned kernel > memory that is physically contiguous. Doing big allocations can turn > into problems during runtime. > > So maybe there is another way? Can we extend the memslot array size > dynamically somehow? Allocate it separately? How much memory does the > memslot array use up with 512 entries? Maybe some rcu protected scheme that doubles the amount of memslots for each overrun? Yes, that would be good and even reduce the footprint for systems with only a small number of memslots. Christian From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: [Qemu-ppc] KVM memory slots limit on powerpc Date: Fri, 04 Sep 2015 12:07:39 +0200 Message-ID: <55E96D6B.2070201@de.ibm.com> References: <55E965FC.8080909@redhat.com> <55E96B87.8020900@de.ibm.com> <55E96CB9.4070100@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: David Gibson , Alex Williamson , Bharata B Rao To: Alexander Graf , Thomas Huth , kvm-ppc@vger.kernel.org, qemu-ppc@nongnu.org, kvm@vger.kernel.org Return-path: Received: from e06smtp11.uk.ibm.com ([195.75.94.107]:34232 "EHLO e06smtp11.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758626AbbIDKHp (ORCPT ); Fri, 4 Sep 2015 06:07:45 -0400 Received: from /spool/local by e06smtp11.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 4 Sep 2015 11:07:43 +0100 In-Reply-To: <55E96CB9.4070100@suse.de> Sender: kvm-owner@vger.kernel.org List-ID: Am 04.09.2015 um 12:04 schrieb Alexander Graf: > > > On 04.09.15 11:59, Christian Borntraeger wrote: >> Am 04.09.2015 um 11:35 schrieb Thomas Huth: >>> >>> Hi all, >>> >>> now that we get memory hotplugging for the spapr machine on qemu-ppc, >>> too, it seems like we easily can hit the amount of KVM-internal memory >>> slots now ("#define KVM_USER_MEM_SLOTS 32" in >>> arch/powerpc/include/asm/kvm_host.h). For example, start >>> qemu-system-ppc64 with a couple of "-device secondary-vga" and "-m >>> 4G,slots=32,maxmem=40G" and then try to hot-plug all 32 DIMMs ... and >>> you'll see that it aborts way earlier already. >>> >>> The x86 code already increased the amount of KVM_USER_MEM_SLOTS to 509 >>> already (+3 internal slots = 512) ... maybe we should now increase the >>> amount of slots on powerpc, too? Since we don't use internal slots on >>> POWER, would 512 be a good value? Or would less be sufficient, too? >> >> When you are at it, the s390 value should also be increased I guess. > > That constant defines the array size for the memslot array in struct kvm > which in turn again gets allocated by kzalloc, so it's pinned kernel > memory that is physically contiguous. Doing big allocations can turn > into problems during runtime. > > So maybe there is another way? Can we extend the memslot array size > dynamically somehow? Allocate it separately? How much memory does the > memslot array use up with 512 entries? Maybe some rcu protected scheme that doubles the amount of memslots for each overrun? Yes, that would be good and even reduce the footprint for systems with only a small number of memslots. Christian