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overcommit: introduce mem-lock=on-fault

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Locking the memory without MCL_ONFAULT instantly prefaults any mmaped
anonymous memory with a write-fault, which introduces a lot of extra
overhead in terms of memory usage when all you want to do is to prevent
kcompactd from migrating and compacting QEMU pages. Add an option to
only lock pages lazily as they're faulted by the process by using
MCL_ONFAULT if asked.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Link: https://lore.kernel.org/r/20250212143920.1269754-5-d-tatianin@yandex-team.ru
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Daniil Tatianin 2025-02-12 17:39:20 +03:00 • committed by Peter Xu
parent cd2e472e54
commit 13057e064a
5 changed files with 45 additions and 14 deletions
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14
qemu-options.hx
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@ -4632,21 +4632,25 @@ SRST
ERST
DEF("overcommit", HAS_ARG, QEMU_OPTION_overcommit,
"-overcommit [mem-lock=on|off][cpu-pm=on|off]\n"
"-overcommit [mem-lock=on|off|on-fault][cpu-pm=on|off]\n"
" run qemu with overcommit hints\n"
" mem-lock=on|off controls memory lock support (default: off)\n"
" mem-lock=on|off|on-fault controls memory lock support (default: off)\n"
" cpu-pm=on|off controls cpu power management (default: off)\n",
QEMU_ARCH_ALL)
SRST
``-overcommit mem-lock=on|off``
``-overcommit mem-lock=on|off|on-fault``
\
``-overcommit cpu-pm=on|off``
Run qemu with hints about host resource overcommit. The default is
to assume that host overcommits all resources.
Locking qemu and guest memory can be enabled via ``mem-lock=on``
(disabled by default). This works when host memory is not
overcommitted and reduces the worst-case latency for guest.
or ``mem-lock=on-fault`` (disabled by default). This works when
host memory is not overcommitted and reduces the worst-case latency for
guest. The on-fault option is better for reducing the memory footprint
since it makes allocations lazy, but the pages still get locked in place
once faulted by the guest or QEMU. Note that the two options are mutually
exclusive.
Guest ability to manage power state of host cpus (increasing latency
for other processes on the same host cpu, but decreasing latency for