Serious issues with xenpaging

Serious issues with xenpaging

[Russell Pavlicek]

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On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:

---
19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?

22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)

22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
[x] largely untested
[x] docs outdated
[x] syntax+logic changed
[x] broken
---

[I've taken the liberty of removing the colorful expletive from the final post]

Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.

Thanks,

Russ Pavlicek
Xen Project Evangelist, Citrix Systems
Home Office: +1-301-829-5327
Mobile: +1-240-397-0199
UK VoIP: +44 1223 852 894

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Re: Serious issues with xenpaging

[Konrad Rzeszutek Wilk]

On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
> On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
> 
> ---
> 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
> 
> 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
> 
> 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
> [x] largely untested
> [x] docs outdated
> [x] syntax+logic changed
> [x] broken
> ---
> 
> [I've taken the liberty of removing the colorful expletive from the final post]
> 
> Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.

The Maintainers file implies otherwise. Let me CC the maintainers.

> 
> Thanks,
> 
> Russ Pavlicek
> Xen Project Evangelist, Citrix Systems
> Home Office: +1-301-829-5327
> Mobile: +1-240-397-0199
> UK VoIP: +44 1223 852 894

> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

Re: Serious issues with xenpaging

[Tim Deegan]

At 10:33 -0500 on 31 Dec (1388482410), Konrad Rzeszutek Wilk wrote:
> On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
> > On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
> > 
> > ---
> > 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
> > 
> > 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
> > 
> > 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
> > [x] largely untested
> > [x] docs outdated
> > [x] syntax+logic changed
> > [x] broken
> > ---
> > 
> > [I've taken the liberty of removing the colorful expletive from the final post]
> > 
> > Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.
> 
> The Maintainers file implies otherwise. Let me CC the maintainers.

Andres really owns this code, so I'll punt to him for an official
answer, but:

- It's been listed as a 'tech preview' on the feature list since it went
  in.  http://wiki.xenproject.org/wiki/Xen_Release_Features says:
  "Preview, due to limited tools support. Hypervisor side in good shape."

- I can't say anything about SuSE's apparent support for it, except
  that ISTR Olaf worked at/for/with SuSE at the time.

- Patches would, of course, be welcome.

Tim.

Re: Serious issues with xenpaging

[Andres Lagar-Cavilla]

On Dec 31, 2013, at 11:31 AM, Tim Deegan <tim@xen.org> wrote:

> At 10:33 -0500 on 31 Dec (1388482410), Konrad Rzeszutek Wilk wrote:
>> On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
>>> On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
>>> 
>>> ---
>>> 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
>>> 
>>> 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
>>> 
>>> 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
>>> [x] largely untested
>>> [x] docs outdated
>>> [x] syntax+logic changed
>>> [x] broken
>>> ---
>>> 
>>> [I've taken the liberty of removing the colorful expletive from the final post]
>>> 
>>> Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.
>> 
>> The Maintainers file implies otherwise. Let me CC the maintainers.
> 
> Andres really owns this code, so I'll punt to him for an official
> answer, but:
The part actively maintained is the hypervisor support for paging, and the interface.

tools/xenpaging is one way to consume that interface. It seems to have suffered from bitrot.

So other than echoing Tim's points below, I'll add

- Some interesting ideas thrown around by Florian in his notes. Could lead to a robust discussion in xen-devel … if Florian is still interested.

- Perhaps the developers who are interested (myself included) should make a decent effort at improving the in-tree tools. There is the argument that for example KSM gives KVM users a sharing solution that just works, whether you like the results or not. In that vein xenpaging apparently doesn't cut it, nor the absence of a basic sharing tool.

One simple paging tool could be lazy restore. There is some interest out there, it would be relatively straightforward to codify.

Andres
> 
> - It's been listed as a 'tech preview' on the feature list since it went
>  in.  http://wiki.xenproject.org/wiki/Xen_Release_Features says:
>  "Preview, due to limited tools support. Hypervisor side in good shape."
> 
> - I can't say anything about SuSE's apparent support for it, except
>  that ISTR Olaf worked at/for/with SuSE at the time.
> 
> - Patches would, of course, be welcome.
> 
> Tim.

Re: Serious issues with xenpaging

[Konrad Rzeszutek Wilk]

On Fri, Jan 03, 2014 at 09:49:36AM -0500, Andres Lagar-Cavilla wrote:
> 
> On Dec 31, 2013, at 11:31 AM, Tim Deegan <tim@xen.org> wrote:
> 
> > At 10:33 -0500 on 31 Dec (1388482410), Konrad Rzeszutek Wilk wrote:
> >> On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
> >>> On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
> >>> 
> >>> ---
> >>> 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
> >>> 
> >>> 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
> >>> 
> >>> 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
> >>> [x] largely untested
> >>> [x] docs outdated
> >>> [x] syntax+logic changed
> >>> [x] broken
> >>> ---
> >>> 
> >>> [I've taken the liberty of removing the colorful expletive from the final post]
> >>> 
> >>> Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.
> >> 
> >> The Maintainers file implies otherwise. Let me CC the maintainers.
> > 
> > Andres really owns this code, so I'll punt to him for an official
> > answer, but:
> The part actively maintained is the hypervisor support for paging, and the interface.
> 
> tools/xenpaging is one way to consume that interface. It seems to have suffered from bitrot.

What is the other interface? Thanks!
> 
> So other than echoing Tim's points below, I'll add
> 
> - Some interesting ideas thrown around by Florian in his notes. Could lead to a robust discussion in xen-devel … if Florian is still interested.
> 
> - Perhaps the developers who are interested (myself included) should make a decent effort at improving the in-tree tools. There is the argument that for example KSM gives KVM users a sharing solution that just works, whether you like the results or not. In that vein xenpaging apparently doesn't cut it, nor the absence of a basic sharing tool.
> 
> One simple paging tool could be lazy restore. There is some interest out there, it would be relatively straightforward to codify.
> 
> Andres
> > 
> > - It's been listed as a 'tech preview' on the feature list since it went
> >  in.  http://wiki.xenproject.org/wiki/Xen_Release_Features says:
> >  "Preview, due to limited tools support. Hypervisor side in good shape."
> > 
> > - I can't say anything about SuSE's apparent support for it, except
> >  that ISTR Olaf worked at/for/with SuSE at the time.
> > 
> > - Patches would, of course, be welcome.
> > 
> > Tim.
> 

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Re: Serious issues with xenpaging

[Andres Lagar-Cavilla]

On Jan 3, 2014, at 1:41 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:

> On Fri, Jan 03, 2014 at 09:49:36AM -0500, Andres Lagar-Cavilla wrote:
>> 
>> On Dec 31, 2013, at 11:31 AM, Tim Deegan <tim@xen.org> wrote:
>> 
>>> At 10:33 -0500 on 31 Dec (1388482410), Konrad Rzeszutek Wilk wrote:
>>>> On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
>>>>> On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
>>>>> 
>>>>> ---
>>>>> 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
>>>>> 
>>>>> 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
>>>>> 
>>>>> 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
>>>>> [x] largely untested
>>>>> [x] docs outdated
>>>>> [x] syntax+logic changed
>>>>> [x] broken
>>>>> ---
>>>>> 
>>>>> [I've taken the liberty of removing the colorful expletive from the final post]
>>>>> 
>>>>> Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.
>>>> 
>>>> The Maintainers file implies otherwise. Let me CC the maintainers.
>>> 
>>> Andres really owns this code, so I'll punt to him for an official
>>> answer, but:
>> The part actively maintained is the hypervisor support for paging, and the interface.
>> 
>> tools/xenpaging is one way to consume that interface. It seems to have suffered from bitrot.
> 
> What is the other interface? Thanks!

Not sure what the question is. There is one interface. What I was referring to, is that tools/xenpaging implements one specific paging policy: victim selection, rate limiting, paging target, all of these are algorithms that entirely define what bang for your money you will get.

Andres
>> 
>> So other than echoing Tim's points below, I'll add
>> 
>> - Some interesting ideas thrown around by Florian in his notes. Could lead to a robust discussion in xen-devel … if Florian is still interested.
>> 
>> - Perhaps the developers who are interested (myself included) should make a decent effort at improving the in-tree tools. There is the argument that for example KSM gives KVM users a sharing solution that just works, whether you like the results or not. In that vein xenpaging apparently doesn't cut it, nor the absence of a basic sharing tool.
>> 
>> One simple paging tool could be lazy restore. There is some interest out there, it would be relatively straightforward to codify.
>> 
>> Andres
>>> 
>>> - It's been listed as a 'tech preview' on the feature list since it went
>>> in.  http://wiki.xenproject.org/wiki/Xen_Release_Features says:
>>> "Preview, due to limited tools support. Hypervisor side in good shape."
>>> 
>>> - I can't say anything about SuSE's apparent support for it, except
>>> that ISTR Olaf worked at/for/with SuSE at the time.
>>> 
>>> - Patches would, of course, be welcome.
>>> 
>>> Tim.
>> 

Re: Serious issues with xenpaging

[Konrad Rzeszutek Wilk]

On Fri, Jan 03, 2014 at 02:51:14PM -0500, Andres Lagar-Cavilla wrote:
> On Jan 3, 2014, at 1:41 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> 
> > On Fri, Jan 03, 2014 at 09:49:36AM -0500, Andres Lagar-Cavilla wrote:
> >> 
> >> On Dec 31, 2013, at 11:31 AM, Tim Deegan <tim@xen.org> wrote:
> >> 
> >>> At 10:33 -0500 on 31 Dec (1388482410), Konrad Rzeszutek Wilk wrote:
> >>>> On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
> >>>>> On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
> >>>>> 
> >>>>> ---
> >>>>> 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
> >>>>> 
> >>>>> 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
> >>>>> 
> >>>>> 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
> >>>>> [x] largely untested
> >>>>> [x] docs outdated
> >>>>> [x] syntax+logic changed
> >>>>> [x] broken
> >>>>> ---
> >>>>> 
> >>>>> [I've taken the liberty of removing the colorful expletive from the final post]
> >>>>> 
> >>>>> Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.
> >>>> 
> >>>> The Maintainers file implies otherwise. Let me CC the maintainers.
> >>> 
> >>> Andres really owns this code, so I'll punt to him for an official
> >>> answer, but:
> >> The part actively maintained is the hypervisor support for paging, and the interface.
> >> 
> >> tools/xenpaging is one way to consume that interface. It seems to have suffered from bitrot.
> > 
> > What is the other interface? Thanks!
> 
> Not sure what the question is. There is one interface. What I was referring to, is that tools/xenpaging implements one specific paging policy: victim selection, rate limiting, paging target, all of these are algorithms that entirely define what bang for your money you will get.
> 

Right, but there is other code that uses this interface as well correct?
Is it available for users ?

Re: Serious issues with xenpaging

[Andres Lagar-Cavilla]

On Jan 3, 2014, at 3:31 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:

> On Fri, Jan 03, 2014 at 02:51:14PM -0500, Andres Lagar-Cavilla wrote:
>> On Jan 3, 2014, at 1:41 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
>> 
>>> On Fri, Jan 03, 2014 at 09:49:36AM -0500, Andres Lagar-Cavilla wrote:
>>>> 
>>>> On Dec 31, 2013, at 11:31 AM, Tim Deegan <tim@xen.org> wrote:
>>>> 
>>>>> At 10:33 -0500 on 31 Dec (1388482410), Konrad Rzeszutek Wilk wrote:
>>>>>> On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
>>>>>>> On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
>>>>>>> 
>>>>>>> ---
>>>>>>> 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
>>>>>>> 
>>>>>>> 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
>>>>>>> 
>>>>>>> 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
>>>>>>> [x] largely untested
>>>>>>> [x] docs outdated
>>>>>>> [x] syntax+logic changed
>>>>>>> [x] broken
>>>>>>> ---
>>>>>>> 
>>>>>>> [I've taken the liberty of removing the colorful expletive from the final post]
>>>>>>> 
>>>>>>> Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.
>>>>>> 
>>>>>> The Maintainers file implies otherwise. Let me CC the maintainers.
>>>>> 
>>>>> Andres really owns this code, so I'll punt to him for an official
>>>>> answer, but:
>>>> The part actively maintained is the hypervisor support for paging, and the interface.
>>>> 
>>>> tools/xenpaging is one way to consume that interface. It seems to have suffered from bitrot.
>>> 
>>> What is the other interface? Thanks!
>> 
>> Not sure what the question is. There is one interface. What I was referring to, is that tools/xenpaging implements one specific paging policy: victim selection, rate limiting, paging target, all of these are algorithms that entirely define what bang for your money you will get.
>> 
> 
> Right, but there is other code that uses this interface as well correct?
> Is it available for users ?

That I know of, Gridcentric's product. It's available as proprietary software for a fee. I am unaware of a sharing user other than Gridcentric. Virtuata was a mem-event user, Gridcentric is, and others have surfaced on the list (Razvan Cocajaru for instance).

Andres

Re: Serious issues with xenpaging

[Olaf Hering]

On Mon, Dec 23, Russell Pavlicek wrote:

> 22-Dec: [S:@:S]lars_kurth [S:@:S]RCPavlicek Hey guys, I wrote down as much as I
> could https://piratenpad.de/p/Ik3lOBLniq1L5TEM    (since I'm on holiday and not
> constant online)


> [I've taken the liberty of removing the colorful expletive from the final post]

Thanks for that.

Quoting the above document:

> Docs issues:
> Most accessible documentation is from SUSE manual
> 
>      written for Xen 4.1/xm toolstack

The work was done for SLES11SP2.

>     xenpaging at least on alpine isn't in $PATH,

Yes, its a helper for the toolstack, just like qemu-dm.
 
>     actual path /usr/lib/xen/bin/xenpaging is weird

Its not because its not supposed to be called manually.
 
> Google turns up the SLES docs, the commit and the "how does one test this" thread
> SLES docs don't mention experimental, but mailing list threads etc, do.

I did some testing during that time and xenpaging works well enough with SP2
and SP3 and its xend based toolstack.

> two years after release it would be a good time to have docs + testing sorted

Yes, but that did not happen due to lack of time, resources and lack of
integration into libxl.

> Looking for sources (last time, at least) doesn't show acticity post-2012
> issues:
>  incoherent:       
> 
>     specify a KB "size" of the paging region
> 
>     but specify a number of pages to hold back
> 
> Decide for one thing, not two. Actually, let us specify any of it?

> I bet the system page size is queryable, so why not KB/MB/GB

The current pagesize can be obtained by syscalls.
Its up to the toolstack to drive xenpaging, so the actual admin
interface should be at this level. The values passed to the helper are
an internal detail of the toolstack.

> /var/lib/xen/xenpaging Path is a bit weird, too. 
> Should it rather be /var/lib/xen/paging in line with /var/lib/xen/save ...

Thats a minor detail. I decided on a default path, so lets stick with
it.

> Where's docs how to define it in domU.cfg for XL stack?

There are no docs because there is no code and not even a usable
proposal how to specify the memory related properties for a domU. What
exists today is either populate-on-demand or a fixed amount of memory.
There was some discussion about two years ago how paging, sharing, PoD
and maybe even tmem could be described in a domU.cfg. Nothing was
decided.

> Sles docs say domU.cfg for xm stack was:
> xenpaging = NUMPAGES
> Sles docs basically say for cli use post-domU-creation
> xenpaging domID NUM
> Actual usage is:
> /usr/lib/xen/bin/xenpaging -f /var/lib/xen/xenpaging/freebsd2.page -
> d 79  -m 524288 

I dont think so. The docs say 'memory=N ; actmem=M' and mem-swap-target
to adjust actmem at runtime.
Old upstream docs/misc/xenpaging.txt is certainly wrong, simply because
no code exists to integrate it into the toolstack. 4.2+ is slightly
better.

> Q's:
> - Obviously, the file name handling when not manually setting it up

That did not parse.

> - How does one pull an overview of current xenpaging consumers?

xenpaging is not called by anything.

> - Is the paging file autodiscarded at domU termination?

It is supposed to be removed, xenpaging will get an event when the domid
disappears. In my testing this part works well.

> Testing / doc:
> - what happens if someone messes up and overcommits the xenpaging area

What does that question mean? Does it mean lack of diskspace in dom0?

> - is there any protection against it?
> - will it block or crash?

The overall OOM handling is not very good. Once a guest starts the full
amount of memory for a given domU is required. So if the whole system is
in overcommit state and all guests do a restart an OOM situation occurs
and some guests will not start anymore due to lack of memory.

> Nested paging:
> Would be nice to document it's needed. This should be the like line 2
> of the docs actually. 

What is "nested paging"?

> Same goes for a note that it's only for HVM domUs, btw.

Thats written down in docs/misc/xenpaging.txt.

> Us users don't really use HVM whenever possible, so this is an
> relevant info. It's moot to see the cool feature, mess around long
> since NODOCS and then find out the one use case you got won't work.
> Would be nicer to have s/w workaround for nested page tables if it's
> not available.  Why? On more modern (basically DDR3-Era) hosts we tend
> to be CPU-bound these days and have TONs of Ram.  For that I'd rather
> use tmem to blink out dup's and have compression fancyness instead of
> paging. (if that were tested & documented and known stable ;) If you'd
> use xenpaging on a current vm host you'd probably topside it if the
> memory is ever really allocated.  (think, 128GB ram, 16GB paging,
> allocate 16GB ... ouch, and it just makes no sense since RAM and
> Raid-1 SSD prices are not much different) It's the 4-core 8GB boxes
> and embedded where the paging is a good last resort (methinks) - but
> there you don't have nested page tables.

xenpaging is not a solution but rather a workaround. It can be used to
quickly swapout parts of a running guest to free host memory. Its not
predictable what pages a guest will access next, so it comes with a high
performance cost (even if the pagefile is in dom0 /dev/shm/).

Olaf

Re: Serious issues with xenpaging

[Konrad Rzeszutek Wilk]

On Fri, Jan 03, 2014 at 04:17:43PM -0500, Andres Lagar-Cavilla wrote:
> On Jan 3, 2014, at 3:31 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> 
> > On Fri, Jan 03, 2014 at 02:51:14PM -0500, Andres Lagar-Cavilla wrote:
> >> On Jan 3, 2014, at 1:41 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> >> 
> >>> On Fri, Jan 03, 2014 at 09:49:36AM -0500, Andres Lagar-Cavilla wrote:
> >>>> 
> >>>> On Dec 31, 2013, at 11:31 AM, Tim Deegan <tim@xen.org> wrote:
> >>>> 
> >>>>> At 10:33 -0500 on 31 Dec (1388482410), Konrad Rzeszutek Wilk wrote:
> >>>>>> On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
> >>>>>>> On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
> >>>>>>> 
> >>>>>>> ---
> >>>>>>> 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
> >>>>>>> 
> >>>>>>> 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
> >>>>>>> 
> >>>>>>> 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
> >>>>>>> [x] largely untested
> >>>>>>> [x] docs outdated
> >>>>>>> [x] syntax+logic changed
> >>>>>>> [x] broken
> >>>>>>> ---
> >>>>>>> 
> >>>>>>> [I've taken the liberty of removing the colorful expletive from the final post]
> >>>>>>> 
> >>>>>>> Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.
> >>>>>> 
> >>>>>> The Maintainers file implies otherwise. Let me CC the maintainers.
> >>>>> 
> >>>>> Andres really owns this code, so I'll punt to him for an official
> >>>>> answer, but:
> >>>> The part actively maintained is the hypervisor support for paging, and the interface.
> >>>> 
> >>>> tools/xenpaging is one way to consume that interface. It seems to have suffered from bitrot.
> >>> 
> >>> What is the other interface? Thanks!
> >> 
> >> Not sure what the question is. There is one interface. What I was referring to, is that tools/xenpaging implements one specific paging policy: victim selection, rate limiting, paging target, all of these are algorithms that entirely define what bang for your money you will get.
> >> 
> > 
> > Right, but there is other code that uses this interface as well correct?
> > Is it available for users ?
> 
> That I know of, Gridcentric's product. It's available as proprietary software for a fee. I am unaware of a sharing user other than Gridcentric. Virtuata was a mem-event user, Gridcentric is, and others have surfaced on the list (Razvan Cocajaru for instance).

In the context of http://wiki.xenproject.org/wiki/GSoc_2014#Lazy_restore_using_memory_paging
should that be removed then? As the dependency of it would be to first 'un-bitrot' it and that
might take more than the original GSoC problem statement describes??

> 
> Andres
> 

Re: Serious issues with xenpaging

[Andres Lagar-Cavilla]

On Mar 7, 2014, at 10:43 AM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:

> On Fri, Jan 03, 2014 at 04:17:43PM -0500, Andres Lagar-Cavilla wrote:
>> On Jan 3, 2014, at 3:31 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
>> 
>>> On Fri, Jan 03, 2014 at 02:51:14PM -0500, Andres Lagar-Cavilla wrote:
>>>> On Jan 3, 2014, at 1:41 PM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
>>>> 
>>>>> On Fri, Jan 03, 2014 at 09:49:36AM -0500, Andres Lagar-Cavilla wrote:
>>>>>> 
>>>>>> On Dec 31, 2013, at 11:31 AM, Tim Deegan <tim@xen.org> wrote:
>>>>>> 
>>>>>>> At 10:33 -0500 on 31 Dec (1388482410), Konrad Rzeszutek Wilk wrote:
>>>>>>>> On Mon, Dec 23, 2013 at 06:34:55PM +0000, Russell Pavlicek wrote:
>>>>>>>>> On Twitter, Florian Heigl sent a out a few messages about issues with xenpaging:
>>>>>>>>> 
>>>>>>>>> ---
>>>>>>>>> 19-Dec: Anyone successfully use #xen<https://twitter.com/search?q=%23xen&src=hash> #xenpaging<https://twitter.com/search?q=%23xenpaging&src=hash>? docs are at SLES manual, rest is mostly this: http://www.gossamer-threads.com/lists/xen/devel/255798<http://t.co/P36VdL84Et> dead feature or usable?
>>>>>>>>> 
>>>>>>>>> 22-Dec: @lars_kurth<https://twitter.com/lars_kurth> @RCPavlicek<https://twitter.com/RCPavlicek> Hey guys, I wrote down as much as I could https://piratenpad.de/p/Ik3lOBLniq1L5TEM   <https://t.co/e5LQCUD9d0> (since I'm on holiday and not constant online)
>>>>>>>>> 
>>>>>>>>> 22-Dec: Yay, tested #xen<https://twitter.com/search?q=%23xen&src=hash> Xenpaging (memory overcommit)
>>>>>>>>> [x] largely untested
>>>>>>>>> [x] docs outdated
>>>>>>>>> [x] syntax+logic changed
>>>>>>>>> [x] broken
>>>>>>>>> ---
>>>>>>>>> 
>>>>>>>>> [I've taken the liberty of removing the colorful expletive from the final post]
>>>>>>>>> 
>>>>>>>>> Is Florian's assessment correct, or is there somewhere we can point him for help?  I'm on vacation this week, but if someone replies to me, I will try to forward the information appropriately.
>>>>>>>> 
>>>>>>>> The Maintainers file implies otherwise. Let me CC the maintainers.
>>>>>>> 
>>>>>>> Andres really owns this code, so I'll punt to him for an official
>>>>>>> answer, but:
>>>>>> The part actively maintained is the hypervisor support for paging, and the interface.
>>>>>> 
>>>>>> tools/xenpaging is one way to consume that interface. It seems to have suffered from bitrot.
>>>>> 
>>>>> What is the other interface? Thanks!
>>>> 
>>>> Not sure what the question is. There is one interface. What I was referring to, is that tools/xenpaging implements one specific paging policy: victim selection, rate limiting, paging target, all of these are algorithms that entirely define what bang for your money you will get.
>>>> 
>>> 
>>> Right, but there is other code that uses this interface as well correct?
>>> Is it available for users ?
>> 
>> That I know of, Gridcentric's product. It's available as proprietary software for a fee. I am unaware of a sharing user other than Gridcentric. Virtuata was a mem-event user, Gridcentric is, and others have surfaced on the list (Razvan Cocajaru for instance).
> 
> In the context of http://wiki.xenproject.org/wiki/GSoc_2014#Lazy_restore_using_memory_paging
> should that be removed then? As the dependency of it would be to first 'un-bitrot' it and that
> might take more than the original GSoC problem statement describes??

My take is that whatever xenpaging un-bitroting may be needed, it will be completely independent of the GSOC project. One objective of this GSOC project is to show another way to put men paging to use. I envision this project to be entirely user-space dom0 code, and to not rely on any existing tools/xenpaging code (ok, maybe refactor the set up of the paging event ring).

Does that make sense?

Thanks
Andres

> 
>> 
>> Andres
>> 

Re: Serious issues with xenpaging

[Olaf Hering]

On Fri, Mar 07, Konrad Rzeszutek Wilk wrote:

> In the context of http://wiki.xenproject.org/wiki/GSoc_2014#Lazy_restore_using_memory_paging
> should that be removed then? As the dependency of it would be to first 'un-bitrot' it and that
> might take more than the original GSoC problem statement describes??

While its a good idea, it can HVM only at this point because PV doesnt
do paging. So its real work to get this done.

Olaf

Re: Serious issues with xenpaging

[Andres Lagar-Cavilla]

On Mar 7, 2014, at 10:54 AM, Olaf Hering <olaf@aepfle.de> wrote:

> On Fri, Mar 07, Konrad Rzeszutek Wilk wrote:
> 
>> In the context of http://wiki.xenproject.org/wiki/GSoc_2014#Lazy_restore_using_memory_paging
>> should that be removed then? As the dependency of it would be to first 'un-bitrot' it and that
>> might take more than the original GSoC problem statement describes??
> 
> While its a good idea, it can HVM only at this point because PV doesnt
> do paging. So its real work to get this done.

Agreed. And certainly no gimme. But they have a couple months to sink into this.

Thanks
Andres
> 
> Olaf