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

[David Hildenbrand]

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.

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.

... but then, management tools can simply start QEMU with "-S", start an 
own timer, and zap QEMU if it didn't manage to come up in time, and 
simply start a new QEMU instance without preallocation enabled.

The "good" thing about that approach is that it will also cover any 
implicit memory preallocation, like using mlock() or VFIO, that don't 
run in ordinary per-hostmem preallocation context. If setting QEMU up 
takes to long, you might want to try on a different hypervisor in your 
cluster instead.


I don't immediately see why we want to make our preallcoation+hostmem 
implementation in QEMU more complicated for such a use case.

--
Thanks,

David / dhildenb