Re: [PATCH v0 0/4] backends/hostmem: add an ability to specify prealloc timeout

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jan 23, 2023 at 04:30:03PM +0300, Daniil Tatianin wrote:
> On 1/23/23 11:57 AM, David Hildenbrand wrote:
> > On 20.01.23 14:47, Daniil Tatianin wrote:
> > > This series introduces new qemu_prealloc_mem_with_timeout() api,
> > > which allows limiting the maximum amount of time to be spent on memory
> > > preallocation. It also adds prealloc statistics collection that is
> > > exposed via an optional timeout handler.
> > > 
> > > This new api is then utilized by hostmem for guest RAM preallocation
> > > controlled via new object properties called 'prealloc-timeout' and
> > > 'prealloc-timeout-fatal'.
> > > 
> > > This is useful for limiting VM startup time on systems with
> > > unpredictable page allocation delays due to memory fragmentation or the
> > > backing storage. The timeout can be configured to either simply emit a
> > > warning and continue VM startup without having preallocated the entire
> > > guest RAM or just abort startup entirely if that is not acceptable for
> > > a specific use case.
> > 
> > The major use case for preallocation is memory resources that cannot be
> > overcommitted (hugetlb, file blocks, ...), to avoid running out of such
> > resources later, while the guest is already running, and crashing it.
> 
> Wouldn't you say that preallocating memory for the sake of speeding up guest
> kernel startup & runtime is a valid use case of prealloc? This way we can
> avoid expensive (for a multitude of reasons) page faults that will otherwise
> slow down the guest significantly at runtime and affect the user experience.
> 
> > Allocating only a fraction "because it takes too long" looks quite
> > useless in that (main use-case) context. We shouldn't encourage QEMU
> > users to play with fire in such a way. IOW, there should be no way
> > around "prealloc-timeout-fatal". Either preallocation succeeded and the
> > guest can run, or it failed, and the guest can't run.
> 
> Here we basically accept the fact that e.g with fragmented memory the kernel
> might take a while in a page fault handler especially for hugetlb because of
> page compaction that has to run for every fault.
> 
> This way we can prefault at least some number of pages and let the guest
> fault the rest on demand later on during runtime even if it's slow and would
> cause a noticeable lag.

Rather than treat this as a problem that needs a timeout, can we
restate it as situations need synchronous vs asynchronous
preallocation ?

For the case where we need synchronous prealloc, current QEMU deals
with that. If it doesn't work quickly enough, mgmt can just kill
QEMU already today.

For the case where you would like some prealloc, but don't mind
if it runs without full prealloc, then why not just treat it as an
entirely asynchronous task ? Instead of calling qemu_prealloc_mem
and waiting for it to complete, just spawn a thread to run
qemu_prealloc_mem, so it doesn't block QEMU startup. This will
have minimal maint burden on the existing code, and will avoid
need for mgmt apps to think about what timeout value to give,
which is good because timeouts are hard to get right.

Most of the time that async background prealloc will still finish
before the guest even gets out of the firmware phase, but if it
takes longer it is no big deal. You don't need to quit the prealloc
job early, you just need it to not delay the guest OS boot IIUC.

This impl could be done with the 'prealloc' property turning from
a boolean on/off, to a enum  on/async/off, where 'on' == sync
prealloc. Or add a separate 'prealloc-async' bool property

With regards,
Daniel
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