[Qemu-devel] linux guests and ksm performance

[Qemu-devel] linux guests and ksm performance

[Peter Lieven]

Hi,

i have recently been playing with an old idea (originally in grsecurity
for security reasons) to change
the policy from zero on allocate to zero after free in the linux page
allocator. My concern is that linux
leaves a lot of waste in the physical memory unlike Windows which per
default zeros pages after
they are freed.

I have run some tests and I can confirm some old results that a hardware
Linux machine
is approximately 2-3% slower with zero after free on big compilation jobs.
This might be due
to either the fact that pages are only zeroed on allocate if GFP_ZERO is
set or due to caching
benefits.

However, in a virtual machine I have not observed the above slow down to
that extend
while the benefit of zero after free in a virtualisation environment is
obvious:

1) zero pages can easily be merged by ksm or other technique.
2) zero (dup) pages are a lot faster to transfer in case of migration.

Therefore I would like to hear your thoughts if it would be a good idea to
change
the strategy in the Linux kernel from zero on allocate to zero after free
automatically
if the 'hypervisor' cpu feature is set? Or even have another technique to
tell a linux
guest that ksm is running on the host.

If this is not feasible can someone think of a kernel module / userspace
program that
zeroes out unused pages periodically.

Peter

Re: [Qemu-devel] linux guests and ksm performance

[Stefan Hajnoczi]

On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <pl@dlh.net> wrote:
> However, in a virtual machine I have not observed the above slow down to
> that extend
> while the benefit of zero after free in a virtualisation environment is
> obvious:
>
> 1) zero pages can easily be merged by ksm or other technique.
> 2) zero (dup) pages are a lot faster to transfer in case of migration.

The other approach is a memory page "discard" mechanism - which
obviously requires more code changes than zeroing freed pages.

The advantage is that we don't take the brute-force and CPU intensive
approach of zeroing pages.  It would be like a fine-grained ballooning
feature.

I hope someone will follow up saying this has already been done or
prototyped :).

Stefan

Re: [Qemu-devel] linux guests and ksm performance

[Javier Guerra Giraldez]

On Thu, Feb 23, 2012 at 11:42 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> The other approach is a memory page "discard" mechanism - which
> obviously requires more code changes than zeroing freed pages.
>
> The advantage is that we don't take the brute-force and CPU intensive
> approach of zeroing pages.  It would be like a fine-grained ballooning
> feature.

(disclaimer: i don't know the code, i'm just guessing)

does KVM emulate the MMU? if so, is there any 'unmap page' primitive?

-- 
Javier

Re: [Qemu-devel] linux guests and ksm performance

[peter.lieven@gmail.com]

Stefan Hajnoczi <stefanha@gmail.com> schrieb:

>On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <pl@dlh.net> wrote:
>> However, in a virtual machine I have not observed the above slow down
>to
>> that extend
>> while the benefit of zero after free in a virtualisation environment
>is
>> obvious:
>>
>> 1) zero pages can easily be merged by ksm or other technique.
>> 2) zero (dup) pages are a lot faster to transfer in case of
>migration.
>
>The other approach is a memory page "discard" mechanism - which
>obviously requires more code changes than zeroing freed pages.
>
>The advantage is that we don't take the brute-force and CPU intensive
>approach of zeroing pages.  It would be like a fine-grained ballooning
>feature.
>

I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power.

>I hope someone will follow up saying this has already been done or
>prototyped :).
>
>Stefan

-- 
Diese Nachricht wurde von meinem Android-Mobiltelefon mit K-9 Mail gesendet.

Re: [Qemu-devel] linux guests and ksm performance

[Stefan Hajnoczi]

On Thu, Feb 23, 2012 at 7:08 PM, peter.lieven@gmail.com <pl@dlh.net> wrote:
>
>
>
>
> Stefan Hajnoczi <stefanha@gmail.com> schrieb:
>
>>On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <pl@dlh.net> wrote:
>>> However, in a virtual machine I have not observed the above slow down
>>to
>>> that extend
>>> while the benefit of zero after free in a virtualisation environment
>>is
>>> obvious:
>>>
>>> 1) zero pages can easily be merged by ksm or other technique.
>>> 2) zero (dup) pages are a lot faster to transfer in case of
>>migration.
>>
>>The other approach is a memory page "discard" mechanism - which
>>obviously requires more code changes than zeroing freed pages.
>>
>>The advantage is that we don't take the brute-force and CPU intensive
>>approach of zeroing pages.  It would be like a fine-grained ballooning
>>feature.
>>
>
> I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power.

It's easy to find a scenario where eagerly zeroing pages is wasteful.
Imagine a process that uses all of physical memory.  Once it
terminates the system is going to run processes that only use a small
set of pages.  It's pointless zeroing all those pages if we're not
going to use them anymore.

Stefan

Re: [Qemu-devel] linux guests and ksm performance

[Stefan Hajnoczi]

On Fri, Feb 24, 2012 at 6:41 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Thu, Feb 23, 2012 at 7:08 PM, peter.lieven@gmail.com <pl@dlh.net> wrote:
>>
>>
>>
>>
>> Stefan Hajnoczi <stefanha@gmail.com> schrieb:
>>
>>>On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <pl@dlh.net> wrote:
>>>> However, in a virtual machine I have not observed the above slow down
>>>to
>>>> that extend
>>>> while the benefit of zero after free in a virtualisation environment
>>>is
>>>> obvious:
>>>>
>>>> 1) zero pages can easily be merged by ksm or other technique.
>>>> 2) zero (dup) pages are a lot faster to transfer in case of
>>>migration.
>>>
>>>The other approach is a memory page "discard" mechanism - which
>>>obviously requires more code changes than zeroing freed pages.
>>>
>>>The advantage is that we don't take the brute-force and CPU intensive
>>>approach of zeroing pages.  It would be like a fine-grained ballooning
>>>feature.
>>>
>>
>> I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power.
>
> It's easy to find a scenario where eagerly zeroing pages is wasteful.
> Imagine a process that uses all of physical memory.  Once it
> terminates the system is going to run processes that only use a small
> set of pages.  It's pointless zeroing all those pages if we're not
> going to use them anymore.

Perhaps the middle path is to zero pages but do it after a grace
timeout.  I wonder if this helps eliminate the 2-3% slowdown you
noticed when compiling.

This requires no special host<->guest interfaces for discarding pages.

Stefan

Re: [Qemu-devel] linux guests and ksm performance

[Stefan Hajnoczi]

On Fri, Feb 24, 2012 at 6:53 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Fri, Feb 24, 2012 at 6:41 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
>> On Thu, Feb 23, 2012 at 7:08 PM, peter.lieven@gmail.com <pl@dlh.net> wrote:
>>> Stefan Hajnoczi <stefanha@gmail.com> schrieb:
>>>
>>>>On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <pl@dlh.net> wrote:
>>>>> However, in a virtual machine I have not observed the above slow down
>>>>to
>>>>> that extend
>>>>> while the benefit of zero after free in a virtualisation environment
>>>>is
>>>>> obvious:
>>>>>
>>>>> 1) zero pages can easily be merged by ksm or other technique.
>>>>> 2) zero (dup) pages are a lot faster to transfer in case of
>>>>migration.
>>>>
>>>>The other approach is a memory page "discard" mechanism - which
>>>>obviously requires more code changes than zeroing freed pages.
>>>>
>>>>The advantage is that we don't take the brute-force and CPU intensive
>>>>approach of zeroing pages.  It would be like a fine-grained ballooning
>>>>feature.
>>>>
>>>
>>> I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power.
>>
>> It's easy to find a scenario where eagerly zeroing pages is wasteful.
>> Imagine a process that uses all of physical memory.  Once it
>> terminates the system is going to run processes that only use a small
>> set of pages.  It's pointless zeroing all those pages if we're not
>> going to use them anymore.
>
> Perhaps the middle path is to zero pages but do it after a grace
> timeout.  I wonder if this helps eliminate the 2-3% slowdown you
> noticed when compiling.

Gah, it's too early in the morning.  I don't think this timer actually
makes sense.

Stefan

Re: [Qemu-devel] linux guests and ksm performance

[Peter Lieven]

Am 24.02.2012 um 08:23 schrieb Stefan Hajnoczi:

> On Fri, Feb 24, 2012 at 6:53 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
>> On Fri, Feb 24, 2012 at 6:41 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
>>> On Thu, Feb 23, 2012 at 7:08 PM, peter.lieven@gmail.com <pl@dlh.net> wrote:
>>>> Stefan Hajnoczi <stefanha@gmail.com> schrieb:
>>>> 
>>>>> On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <pl@dlh.net> wrote:
>>>>>> However, in a virtual machine I have not observed the above slow down
>>>>> to
>>>>>> that extend
>>>>>> while the benefit of zero after free in a virtualisation environment
>>>>> is
>>>>>> obvious:
>>>>>> 
>>>>>> 1) zero pages can easily be merged by ksm or other technique.
>>>>>> 2) zero (dup) pages are a lot faster to transfer in case of
>>>>> migration.
>>>>> 
>>>>> The other approach is a memory page "discard" mechanism - which
>>>>> obviously requires more code changes than zeroing freed pages.
>>>>> 
>>>>> The advantage is that we don't take the brute-force and CPU intensive
>>>>> approach of zeroing pages.  It would be like a fine-grained ballooning
>>>>> feature.
>>>>> 
>>>> 
>>>> I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power.
>>> 
>>> It's easy to find a scenario where eagerly zeroing pages is wasteful.
>>> Imagine a process that uses all of physical memory.  Once it
>>> terminates the system is going to run processes that only use a small
>>> set of pages.  It's pointless zeroing all those pages if we're not
>>> going to use them anymore.
>> 
>> Perhaps the middle path is to zero pages but do it after a grace
>> timeout.  I wonder if this helps eliminate the 2-3% slowdown you
>> noticed when compiling.
> 
> Gah, it's too early in the morning.  I don't think this timer actually
> makes sense.

ok, that would be the idea of an ansynchronous page zeroing in the guest. i also
think this is to complicated.

maybe the other idea is too simple:
is it possible to give the guest a hint that ksm is enabled on the host (lets say in
a way like its done with kvmclock). if ksm is enabled on the host the administrator
has already made the decision that performance is not so important and he/she
is eager to save physical memory. what if and only if this flag is set switch from
zero on allocate to zero after free. i think the whole thing is less than 10-20
lines of code. and its code that has been proven to be working well in grsecurity
for ages.

this might introduce a little (2-3%) overhead, but only if there is a lot of non GFP_FREE
memory is allocated, but its definitely faster than swapping. 
of course, it has to be garanteed that this code does not slow down normal systems
due to additionales branches (would it be enough to mark the if statements as unlikely?)

peter


peter





> 
> Stefan

Re: [Qemu-devel] linux guests and ksm performance

[Peter Lieven]

On 24.02.2012 08:23, Stefan Hajnoczi wrote:
> On Fri, Feb 24, 2012 at 6:53 AM, Stefan Hajnoczi<stefanha@gmail.com>  wrote:
>> On Fri, Feb 24, 2012 at 6:41 AM, Stefan Hajnoczi<stefanha@gmail.com>  wrote:
>>> On Thu, Feb 23, 2012 at 7:08 PM, peter.lieven@gmail.com<pl@dlh.net>  wrote:
>>>> Stefan Hajnoczi<stefanha@gmail.com>  schrieb:
>>>>
>>>>> On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven<pl@dlh.net>  wrote:
>>>>>> However, in a virtual machine I have not observed the above slow down
>>>>> to
>>>>>> that extend
>>>>>> while the benefit of zero after free in a virtualisation environment
>>>>> is
>>>>>> obvious:
>>>>>>
>>>>>> 1) zero pages can easily be merged by ksm or other technique.
>>>>>> 2) zero (dup) pages are a lot faster to transfer in case of
>>>>> migration.
>>>>>
>>>>> The other approach is a memory page "discard" mechanism - which
>>>>> obviously requires more code changes than zeroing freed pages.
>>>>>
>>>>> The advantage is that we don't take the brute-force and CPU intensive
>>>>> approach of zeroing pages.  It would be like a fine-grained ballooning
>>>>> feature.
>>>>>
>>>> I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power.
>>> It's easy to find a scenario where eagerly zeroing pages is wasteful.
>>> Imagine a process that uses all of physical memory.  Once it
>>> terminates the system is going to run processes that only use a small
>>> set of pages.  It's pointless zeroing all those pages if we're not
>>> going to use them anymore.
>> Perhaps the middle path is to zero pages but do it after a grace
>> timeout.  I wonder if this helps eliminate the 2-3% slowdown you
>> noticed when compiling.
> Gah, it's too early in the morning.  I don't think this timer actually
> makes sense.
do you think it makes then sense to make a patchset/proposal to notice a 
guest
kernel about the presense of ksm in the host and switch to zero after free?

peter
> Stefan
>

Re: [Qemu-devel] linux guests and ksm performance

[Peter Lieven]

On 28.02.2012 13:05, Stefan Hajnoczi wrote:
> On Tue, Feb 28, 2012 at 11:46 AM, Peter Lieven<pl@dlh.net>  wrote:
>> On 24.02.2012 08:23, Stefan Hajnoczi wrote:
>>> On Fri, Feb 24, 2012 at 6:53 AM, Stefan Hajnoczi<stefanha@gmail.com>
>>>   wrote:
>>>> On Fri, Feb 24, 2012 at 6:41 AM, Stefan Hajnoczi<stefanha@gmail.com>
>>>>   wrote:
>>>>> On Thu, Feb 23, 2012 at 7:08 PM, peter.lieven@gmail.com<pl@dlh.net>
>>>>>   wrote:
>>>>>> Stefan Hajnoczi<stefanha@gmail.com>    schrieb:
>>>>>>
>>>>>>> On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven<pl@dlh.net>    wrote:
>>>>>>>> However, in a virtual machine I have not observed the above slow down
>>>>>>> to
>>>>>>>> that extend
>>>>>>>> while the benefit of zero after free in a virtualisation environment
>>>>>>> is
>>>>>>>> obvious:
>>>>>>>>
>>>>>>>> 1) zero pages can easily be merged by ksm or other technique.
>>>>>>>> 2) zero (dup) pages are a lot faster to transfer in case of
>>>>>>> migration.
>>>>>>>
>>>>>>> The other approach is a memory page "discard" mechanism - which
>>>>>>> obviously requires more code changes than zeroing freed pages.
>>>>>>>
>>>>>>> The advantage is that we don't take the brute-force and CPU intensive
>>>>>>> approach of zeroing pages.  It would be like a fine-grained ballooning
>>>>>>> feature.
>>>>>>>
>>>>>> I dont think that it is cpu intense. All user pages are zeroed anyway,
>>>>>> but at allocation time it shouldnt be a big difference in terms of cpu
>>>>>> power.
>>>>> It's easy to find a scenario where eagerly zeroing pages is wasteful.
>>>>> Imagine a process that uses all of physical memory.  Once it
>>>>> terminates the system is going to run processes that only use a small
>>>>> set of pages.  It's pointless zeroing all those pages if we're not
>>>>> going to use them anymore.
>>>> Perhaps the middle path is to zero pages but do it after a grace
>>>> timeout.  I wonder if this helps eliminate the 2-3% slowdown you
>>>> noticed when compiling.
>>> Gah, it's too early in the morning.  I don't think this timer actually
>>> makes sense.
>>
>> do you think it makes then sense to make a patchset/proposal to notice a
>> guest
>> kernel about the presense of ksm in the host and switch to zero after free?
> I think your idea is interesting - whether or not people are happy
> with it will depend on the performance impact.  It seems reasonable to
> me.
could you support/help me in implementing and publishing this approach?

Peter

Re: [Qemu-devel] linux guests and ksm performance

[Avi Kivity]

On 02/24/2012 08:41 AM, Stefan Hajnoczi wrote:
> >
> > I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power.
>
> It's easy to find a scenario where eagerly zeroing pages is wasteful.
> Imagine a process that uses all of physical memory.  Once it
> terminates the system is going to run processes that only use a small
> set of pages.  It's pointless zeroing all those pages if we're not
> going to use them anymore.

In the long term, we will use them, except if the guest is completely idle.

The scenario in which zeroing is expensive is when the page is refilled
through DMA.  In that case the zeroing was wasted.  This is a pretty
common scenario in pagecache intensive workloads.

-- 
error compiling committee.c: too many arguments to function

Re: [Qemu-devel] linux guests and ksm performance

[Peter Lieven]

On 28.02.2012 14:16, Avi Kivity wrote:
> On 02/24/2012 08:41 AM, Stefan Hajnoczi wrote:
>>> I dont think that it is cpu intense. All user pages are zeroed anyway, but at allocation time it shouldnt be a big difference in terms of cpu power.
>> It's easy to find a scenario where eagerly zeroing pages is wasteful.
>> Imagine a process that uses all of physical memory.  Once it
>> terminates the system is going to run processes that only use a small
>> set of pages.  It's pointless zeroing all those pages if we're not
>> going to use them anymore.
> In the long term, we will use them, except if the guest is completely idle.
>
> The scenario in which zeroing is expensive is when the page is refilled
> through DMA.  In that case the zeroing was wasted.  This is a pretty
> common scenario in pagecache intensive workloads.
>
Avi, what do you think of the proposal to give the guest vm a hint
that the host is running ksm? In that case the administrator
has already chosen that saving physical memory is more important
than performance to him?

Peter

Re: [Qemu-devel] linux guests and ksm performance

[Avi Kivity]

On 02/28/2012 03:20 PM, Peter Lieven wrote:
> On 28.02.2012 14:16, Avi Kivity wrote:
>> On 02/24/2012 08:41 AM, Stefan Hajnoczi wrote:
>>>> I dont think that it is cpu intense. All user pages are zeroed
>>>> anyway, but at allocation time it shouldnt be a big difference in
>>>> terms of cpu power.
>>> It's easy to find a scenario where eagerly zeroing pages is wasteful.
>>> Imagine a process that uses all of physical memory.  Once it
>>> terminates the system is going to run processes that only use a small
>>> set of pages.  It's pointless zeroing all those pages if we're not
>>> going to use them anymore.
>> In the long term, we will use them, except if the guest is completely
>> idle.
>>
>> The scenario in which zeroing is expensive is when the page is refilled
>> through DMA.  In that case the zeroing was wasted.  This is a pretty
>> common scenario in pagecache intensive workloads.
>>
> Avi, what do you think of the proposal to give the guest vm a hint
> that the host is running ksm? In that case the administrator
> has already chosen that saving physical memory is more important
> than performance to him?

It makes some sense.  Perhaps through the balloon device, a flag that
indicates that voluntary ballooning will be gratefully accepted.

-- 
error compiling committee.c: too many arguments to function

Re: [Qemu-devel] linux guests and ksm performance

[Gleb Natapov]

On Thu, Feb 23, 2012 at 04:42:54PM +0000, Stefan Hajnoczi wrote:
> On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <pl@dlh.net> wrote:
> > However, in a virtual machine I have not observed the above slow down to
> > that extend
> > while the benefit of zero after free in a virtualisation environment is
> > obvious:
> >
> > 1) zero pages can easily be merged by ksm or other technique.
> > 2) zero (dup) pages are a lot faster to transfer in case of migration.
> 
> The other approach is a memory page "discard" mechanism - which
> obviously requires more code changes than zeroing freed pages.
> 
> The advantage is that we don't take the brute-force and CPU intensive
> approach of zeroing pages.  It would be like a fine-grained ballooning
> feature.
> 
> I hope someone will follow up saying this has already been done or
> prototyped :).
> 
That was attempted. It is called "page hinting", but AFAIK due to
complex locking issue attempt was abandoned.

--
			Gleb.

Re: [Qemu-devel] linux guests and ksm performance

[Avi Kivity]

On 02/23/2012 06:42 PM, Stefan Hajnoczi wrote:
> On Thu, Feb 23, 2012 at 3:40 PM, Peter Lieven <pl@dlh.net> wrote:
> > However, in a virtual machine I have not observed the above slow down to
> > that extend
> > while the benefit of zero after free in a virtualisation environment is
> > obvious:
> >
> > 1) zero pages can easily be merged by ksm or other technique.
> > 2) zero (dup) pages are a lot faster to transfer in case of migration.
>
> The other approach is a memory page "discard" mechanism - which
> obviously requires more code changes than zeroing freed pages.
>
> The advantage is that we don't take the brute-force and CPU intensive
> approach of zeroing pages.  It would be like a fine-grained ballooning
> feature.
>
> I hope someone will follow up saying this has already been done or
> prototyped :).

It already exists - that's the balloon code.  Right now it's host
driven, but maybe we can modify it to allow the guest to initiate
balloon inflations.

-- 
error compiling committee.c: too many arguments to function