Re: [Xen-devel] [PATCH v2] docs: clarify xl mem-max semantics

[Jim Fehlig <jfehlig@xxxxxxxx>]

On 01/23/2017 02:49 AM, Juergen Gross wrote:
> The information given in the xl man page for the mem-max command is
> rather brief. Expand it in order to let the reader understand what it
> is really doing.
>
> As the related libxl function libxl_domain_setmaxmem() isn't much
> clearer add a comment to it explaining the desired semantics.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> V2: rewording as suggested by Jim Fehlig and Ian Jackson
> ---
>  docs/man/xl.pod.1.in | 10 ++++++++++
>  tools/libxl/libxl.c  |  6 ++++++
>  2 files changed, 16 insertions(+)
>
> diff --git a/docs/man/xl.pod.1.in b/docs/man/xl.pod.1.in
> index 09c1faa..7caed08 100644
> --- a/docs/man/xl.pod.1.in
> +++ b/docs/man/xl.pod.1.in
> @@ -401,6 +401,16 @@ for bytes.
>  The mem-max value may not correspond to the actual memory used in the
>  domain, as it may balloon down its memory to give more back to the OS.
>
> +The value given just sets the memory amount the domain is allowed to allocate
> +in the hypervisor. It can't be set lower than the current reservation, but
> +it is allowed to be higher than the configured maximum memory size of the
> +domain (B<maxmem> parameter in the domain's configuration). Using B<xl mem-max>
> +to set the maximum memory above the initial B<maxmem> value will not allow the
> +additional memory to be used via B<xl mem-set>. The initial B<maxmem> value is
> +still used as an upper limit for B<xl mem-set>.
> +
> +The domain is not receiving any signal regarding the changed memory limit.
> +
>  =item B<mem-set> I<domain-id> I<mem>
>
>  Set the domain's used memory using the balloon driver; append 't' for
> diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
> index 0622311..ed59510 100644
> --- a/tools/libxl/libxl.c
> +++ b/tools/libxl/libxl.c
> @@ -4018,6 +4018,12 @@ out:
>
>  
> /******************************************************************************/
>
> +/*
> + * Set the maximum memory size of the domain in the hypervisor. There is no
> + * change of the current memory size involved. The allowed memory size can
s/allowed/specified/ ? Or perhaps "The value of max_memkb can even..." ? Sorry I 
didn't look closely at this hunk in V1.
> + * even be above the configured maxmem size of the domain, but the related
> + * Xenstore entry memory/static-max isn't modified!
> + */
>  int libxl_domain_setmaxmem(libxl_ctx *ctx, uint32_t domid, uint64_t max_memkb)
>  {
>      GC_INIT(ctx);
I have a related question for you (or Ian, Wei, ...): What does max_pages (as 
seen with 'q' debug key') represent? How is it related to memory/static-max in 
xenstore, if at all?
When a domain is running, libvirt reports maximum memory through max_memkb field 
of libxl_dominfo struct. max_memkb is filled with 
PAGE_TO_MEMKB(xcinfo->max_pages). AFAICT, xcinfo->max_pages contains the same 
value as max_pages from 'q' debug key, but neither of those represent the maxmem 
value used at domain creation. E.g.
# cat test.xl | grep mem
maxmem = 8192
memory = 2048
# xl create test.xl
# xl debug-key q && xl dmesg | grep max_pages
(XEN)     nr_pages=524205 xenheap_pages=6 shared_pages=0 paged_pages=0 
dirty_cpus={20,42} max_pages=524544
On this system, 524544*4096=2049M which is closer to 'memory' than 'maxmem'.

I have a patch which fixes this in libvirt, but I'm having a hard time 
articulating why the current use of max_pages is incorrect :-).
TIA for any comments you might have on that.

Regards,
Jim


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel