Tuning KVM - Revision history

Kenyon: /* CPU Performance */ fix useless use of cat

2018-06-11T21:06:40Z

CPU Performance: fix useless use of cat

← Older revision		Revision as of 17:06, 11 June 2018
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	To see the difference between the capabilities of the host CPU versus the guest, just compare the output of the following commands on each system:		To see the difference between the capabilities of the host CPU versus the guest, just compare the output of the following commands on each system:

	~~cat~~ /proc/cpuinfo ~~\| grep flags~~ \| uniq		grep flags /proc/cpuinfo \| uniq

	For example, the default setting on a 64 bit host machine is "-cpu qemu64".<br /> This includes the following flags:		For example, the default setting on a 64 bit host machine is "-cpu qemu64".<br /> This includes the following flags:
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	See [http://blog.incase.de/index.php/cpu-feature-flags-and-their-meanings/ CPU Feature Flags And Their Meanings] (and other resources on the web) for more information.		See [http://blog.incase.de/index.php/cpu-feature-flags-and-their-meanings/ CPU Feature Flags And Their Meanings] (and other resources on the web) for more information.


	== Networking ==		== Networking ==

Robert: /* Networking */

2015-05-08T08:29:38Z

Networking

← Older revision		Revision as of 04:29, 8 May 2015
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	or		or
	qemu -net nic,model=virtio,mac=... '''-net bridge,br=...,helper=/usr/local/libexec/qemu-bridge-helper'''		qemu -net nic,model=virtio,mac=... '''-net bridge,br=...,helper=/usr/local/libexec/qemu-bridge-helper'''

			Besides virtio, approach of vt-d passthrough give high (even higher) performance as well.

			qemu '''-net none -device vfio-pci,host=...'''

	QEMU also defaults to the RTL8139 network interface card (NIC) model. Again this card is compatible with most guests, but does not offer the best performance. If your guest supports it, switch to the virtio model:		QEMU also defaults to the RTL8139 network interface card (NIC) model. Again this card is compatible with most guests, but does not offer the best performance. If your guest supports it, switch to the virtio model:

Robert: /* Networking */

2015-05-08T08:23:43Z

Networking

← Older revision		Revision as of 04:23, 8 May 2015
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	qemu -net nic,model=virtio,mac=... '''-net tap,ifname=...'''		qemu -net nic,model=virtio,mac=... '''-net tap,ifname=...'''

			or
			qemu -net nic,model=virtio,mac=... '''-net bridge,br=...,helper=/usr/local/libexec/qemu-bridge-helper'''

	QEMU also defaults to the RTL8139 network interface card (NIC) model. Again this card is compatible with most guests, but does not offer the best performance. If your guest supports it, switch to the virtio model:		QEMU also defaults to the RTL8139 network interface card (NIC) model. Again this card is compatible with most guests, but does not offer the best performance. If your guest supports it, switch to the virtio model:

	qemu -net nic,'''model=virtio''',mac=... -net tap,ifname=...		qemu -net nic,'''model=virtio''',mac=... -net tap,ifname=...


	== Storage ==		== Storage ==

AnthonyLiguori at 17:49, 5 September 2012

2012-09-05T17:49:03Z

← Older revision		Revision as of 13:49, 5 September 2012
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	qemu -drive '''file=/dev/mapper/ImagesVolumeGroup-Guest1''',cache=none,if=virtio		qemu -drive '''file=/dev/mapper/ImagesVolumeGroup-Guest1''',cache=none,if=virtio

	QEMU also supports a wide variety of caching modes. ~~Writeback is useful for testing but does not offer storage guarantees. Writethrough (the default) is safer, and relies on the host cache.~~ If you're using raw volumes or partitions, it is best to avoid the cache completely, which reduces data copies and bus traffic:		QEMU also supports a wide variety of caching modes. If you're using raw volumes or partitions, it is best to avoid the cache completely, which reduces data copies and bus traffic:

	qemu -drive file=/dev/mapper/ImagesVolumeGroup-Guest1,'''cache=none''',if=virtio		qemu -drive file=/dev/mapper/ImagesVolumeGroup-Guest1,'''cache=none''',if=virtio

Joerg: no btrfs on the host

2011-08-20T09:43:16Z

no btrfs on the host

← Older revision		Revision as of 05:43, 20 August 2011
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	qemu -drive file=/dev/mapper/ImagesVolumeGroup-Guest1,cache=none,'''if=virtio'''		qemu -drive file=/dev/mapper/ImagesVolumeGroup-Guest1,cache=none,'''if=virtio'''

			Don't use the linux filesystem btrfs on the host for the image files. It will result in low IO performance. The kvm guest may even freeze when high IO traffic is done on the guest.

Jcox at 16:12, 19 January 2010

2010-01-19T16:12:24Z

← Older revision		Revision as of 12:12, 19 January 2010
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	fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx lm up pni hypervisor		fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx lm up pni hypervisor

	The host itself might support other flags like cx16, mmxext, and so on. See [http://blog.incase.de/index.php/cpu-feature-flags-and-their-meanings/ CPU Feature Flags And Their Meanings], and other resources on the web for more information.		The host itself might support other flags like cx16, mmxext, and so on.

			See [http://blog.incase.de/index.php/cpu-feature-flags-and-their-meanings/ CPU Feature Flags And Their Meanings] (and other resources on the web) for more information.


	== Networking ==		== Networking ==
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	qemu -net nic,'''model=virtio''',mac=... -net tap,ifname=...		qemu -net nic,'''model=virtio''',mac=... -net tap,ifname=...


	== Storage ==		== Storage ==

Jcox at 16:10, 19 January 2010

2010-01-19T16:10:24Z

← Older revision		Revision as of 12:10, 19 January 2010
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	qemu '''-cpu qemu64,+ssse3,+sse4.1,+sse4.2,+x2apic'''		qemu '''-cpu qemu64,+ssse3,+sse4.1,+sse4.2,+x2apic'''

			To see the difference between the capabilities of the host CPU versus the guest, just compare the output of the following commands on each system:

			cat /proc/cpuinfo \| grep flags \| uniq

			For example, the default setting on a 64 bit host machine is "-cpu qemu64".<br /> This includes the following flags:

			fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx lm up pni hypervisor

			The host itself might support other flags like cx16, mmxext, and so on. See [http://blog.incase.de/index.php/cpu-feature-flags-and-their-meanings/ CPU Feature Flags And Their Meanings], and other resources on the web for more information.

	== Networking ==		== Networking ==

AviKivity: cpu, networking, storage tuning tips

2009-09-29T17:23:25Z

cpu, networking, storage tuning tips

New page

== CPU Performance ==

Modern processors come with a wide variety of performance enhancing features such as streaming instructions sets (sse) and other performance-enhancing instructions. These features vary from processor to processor.

QEMU and KVM default to a compatible subset of cpu features, so that if you change your host processor, or perform a live migration, the guest will see its cpu features unchanged. This is great for compatibility but comes at a performance cost.

To pass all available host processor features to the guest, use the command line switch

qemu '''-cpu host'''

if you wish to retain compatibility, you can expose selected features to your guest. If all your hosts have these features, compatibility is retained:

qemu '''-cpu qemu64,+ssse3,+sse4.1,+sse4.2,+x2apic'''

== Networking ==

QEMU defaults to user-mode networking (slirp), which is available without prior setup and without administrative privileges on the host. It is also unfortunately very slow. To get high performance networking, switch to a bridged setup via the -net tap command line switches.

qemu -net nic,model=virtio,mac=... '''-net tap,ifname=...'''

QEMU also defaults to the RTL8139 network interface card (NIC) model. Again this card is compatible with most guests, but does not offer the best performance. If your guest supports it, switch to the virtio model:

qemu -net nic,'''model=virtio''',mac=... -net tap,ifname=...

== Storage ==

QEMU supports a wide variety for storage formats and back-ends. Easiest to use are the raw and qcow2 formats, but for the best performance it is best to use a raw partition. You can create either a logical volume or a partition and assign it to the guest:

qemu -drive '''file=/dev/mapper/ImagesVolumeGroup-Guest1''',cache=none,if=virtio

QEMU also supports a wide variety of caching modes. Writeback is useful for testing but does not offer storage guarantees. Writethrough (the default) is safer, and relies on the host cache. If you're using raw volumes or partitions, it is best to avoid the cache completely, which reduces data copies and bus traffic:

qemu -drive file=/dev/mapper/ImagesVolumeGroup-Guest1,'''cache=none''',if=virtio

As with networking, QEMU supports several storage interfaces. The default, IDE, is highly supported by guests but may be slow, especially with disk arrays. If your guest supports it, use the virtio interface:

qemu -drive file=/dev/mapper/ImagesVolumeGroup-Guest1,cache=none,'''if=virtio'''