[libvirt PATCH] daemon: set default memlock limit for systemd service

Michal Prívozník mprivozn at redhat.com
Wed Feb 26 15:33:13 UTC 2020


On 2/26/20 4:07 PM, Pavel Hrdina wrote:
> The default memlock limit is 64k which is not enough to start a single
> VM. The requirements for one VM are 12k, 8k for eBPF map and 4k for eBPF
> program, however, it fails to create eBPF map and program with 64k limit.
> By testing I figured out that the minimal limit is 80k to start a single
> VM with functional eBPF and if I add 12k I can start another one.
> 
> This leads into following calculation:
> 
> 80k as memlock limit worked to start a VM with eBPF which means there
> is 68k of lock memory that I was not able to figure out what was using
> it.  So to get a number for 4096 VMs:
> 
>         68 + 12 * 4096 = 49220
> 
> If we round it up we will get 49M of memory lock limit to support 4096
> VMs with default map size which can hold 64 entries for devices.
> 
> This should be good enough as a sane default and users can change it if
> the need to.
> 
> Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1807090
> 
> Signed-off-by: Pavel Hrdina <phrdina at redhat.com>
> ---
>  src/remote/libvirtd.service.in | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/src/remote/libvirtd.service.in b/src/remote/libvirtd.service.in
> index 9c8c54a2ef..8a3ace5bdb 100644
> --- a/src/remote/libvirtd.service.in
> +++ b/src/remote/libvirtd.service.in
> @@ -40,6 +40,11 @@ LimitNOFILE=8192
>  # A conservative default of 8 tasks per guest results in a TasksMax of
>  # 32k to support 4096 guests.
>  TasksMax=32768
> +# With cgroups v2 there is no devices controller anymore, we have to use
> +# eBPF to control access to devices.  In order to do that we create a eBPF
> +# hash MAP which locked memory.  The default map size for 64 devices together

s/locked/locks/

> +# with program takes 12k per guest which results in 49M to support 4096 guests.
> +LimitMEMLOCK=49M

Should we round this up to the nearest power of two? 49MB looks just
ugly. This is just a limit, it doesn't mean that libvirtd will lock
whole 49MB (or 64MB as I suggest) right from the beginning.

>  
>  [Install]
>  WantedBy=multi-user.target
> 

Michal




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