Re: [libvirt PATCH v3 5/5] qemu: enable asynchronous teardown on s390x hosts by default

On Wed, Jul 05, 2023 at 03:29:39PM +0200, Claudio Imbrenda wrote: > On Wed, 5 Jul 2023 14:08:27 +0100 > Daniel P. Berrangé <berrange(a)redhat.com&gt; wrote: > >> On Wed, Jul 05, 2023 at 02:46:03PM +0200, Claudio Imbrenda wrote: >>> On Wed, 5 Jul 2023 13:26:32 +0100 >>> Daniel P. Berrangé <berrange(a)redhat.com&gt; wrote: >>> >>> [...] >>> >>>>>> I rather think mgmt apps need to explicitly opt-in to async teardown, >>>>>> so they're aware that they need to take account of delayed RAM >>>>>> availablity in their accounting / guest placement logic. >>>>> >>>>> what would you think about enabling it by default only for guests that >>>>> are capable to run in Secure Execution mode? >>>> >>>> IIUC, that's basically /all/ guests if running on new enough hardware >>>> with prot_virt=1 enabled on the host OS, so will still present challenges >>>> to mgmt apps needing to be aware of this behaviour AFAICS. >>> >>> I think there is some fencing still? I don't think it's automatic >> >> IIUC, the following sequence is possible >> >> 1. Start QEMU with -m 500G >> -> QEMU spawns async teardown helper process >> 2. Stop QEMU >> -> Async teardown helper process remains running while > > not running, the process terminates immediately as soon as QEMU > terminates. the termination takes some time, because of the memory > cleanup. > >> kernel releases RAM >> 3. Start QEMU with -m 500G >> -> Fails with ENOMEM > > why though? the new VM will not manage to instantly use all of the > memory > >> ...time passes... >> 4. Async teardown helper finally terminates >> -> The full original 500G is only now released for use > > memory starts to get freed as soon as the helper process terminates > (which is as immediately as possible after QEMU terminates > > so unless you have a guest that will allocate and use all of its memory > immediately as fast as possible at boot, this won't be a concern. When using huge pages, QEMU should be fully allocating memory immediately, regardless of whether the guest OS touches all RAM.