Re: [Xen-devel] [RFC PATCH] mm, hotplug: get rid of auto_online_blocks

[Michal Hocko <mhocko@xxxxxxxxxx>]

On Mon 13-03-17 14:42:37, Vitaly Kuznetsov wrote:
> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> 
> > On Mon 13-03-17 13:54:59, Vitaly Kuznetsov wrote:
> >> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> >> 
> >> > On Mon 13-03-17 11:55:54, Igor Mammedov wrote:
> >> >> > > 
> >> >> > >        - suggested RFC is not acceptable from virt point of view
> >> >> > >          as it regresses guests on top of x86 kvm/vmware which
> >> >> > >          both use ACPI based memory hotplug.
> >> >> > > 
> >> >> > >        - udev/userspace solution doesn't work in practice as it's
> >> >> > >          too slow and unreliable when system is under load which
> >> >> > >          is quite common in virt usecase. That's why auto online
> >> >> > >          has been introduced in the first place.  
> >> >> > 
> >> >> > Please try to be more specific why "too slow" is a problem. Also how
> >> >> > much slower are we talking about?
> >> >>
> >> >> In virt case on host with lots VMs, userspace handler
> >> >> processing could be scheduled late enough to trigger a race
> >> >> between (guest memory going away/OOM handler) and memory
> >> >> coming online.
> >> >
> >> > Either you are mixing two things together or this doesn't really make
> >> > much sense. So is this a balloning based on memory hotplug (aka active
> >> > memory hotadd initiated between guest and host automatically) or a guest
> >> > asking for additional memory by other means (pay more for memory etc.)?
> >> > Because if this is an administrative operation then I seriously question
> >> > this reasoning.
> >> 
> >> I'm probably repeating myself but it seems this point was lost:
> >> 
> >> This is not really a 'ballooning', it is just a pure memory
> >> hotplug. People may have any tools monitoring their VM memory usage and
> >> when a VM is running low on memory they may want to hotplug more memory
> >> to it.
> >
> > What is the API those guests ask for the memory? And who is actually
> > responsible to ask for that memory? Is it a kernel or userspace
> > solution?
> 
> Whatever, this can even be a system administrator running
> 'free'.

I am pretty sure that 'free' will not give you additional memory but
let's be serious here... If this is solely about monitoring from
userspace and requesting more memory from the userspace then I would
consider arguing about timely hotplug operation as void because there is
absolutely no guarantee to do the request itself in a timely fashion.

> Hyper-V driver sends si_mem_available() and
> vm_memory_committed() metrics to the host every second and this can be
> later queried by any tool (e.g. powershell script).

And how exactly is this related to the acpi hotplug which you were
arguing needs the timely handling as well?
 
> >> With udev-style memory onlining they should be aware of page
> >> tables and other in-kernel structures which require allocation so they
> >> need to add memory slowly and gradually or they risk running into OOM
> >> (at least getting some processes killed and these processes may be
> >> important). With in-kernel memory hotplug everything happens
> >> synchronously and no 'slowly and gradually' algorithm is required in
> >> all tools which may trigger memory hotplug.
> >
> > What prevents those APIs being used reasonably and only asks so much
> > memory as they can afford? I mean 1.5% available memory necessary for
> > the hotplug is not all that much. Or more precisely what prevents to ask
> > for this additional memory in a synchronous way?
> 
> The knowledge about the fact that we need to add memory slowly and
> wait till it gets onlined is not obvious.

yes it is and we cannot afford to give a better experience with the
implementation that requires to have memory to online a memory.

> AFAIR when you hotplug memory
> to Windows VMs there is no such thing as 'onlining', and no brain is
> required, a simple script 'low memory -> add mory memory' always
> works. Asking all these script writers to think twice before issuing a
> memory add command memory sounds like too much (to me).

Pardon me, but not requiring a brain while doing something on Windows
VMs is not really an argument...
-- 
Michal Hocko
SUSE Labs

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