Re: [Xen-devel] xl only waits 33 seconds for ballooning to complete

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Jan 8, 2015 at 1:11 AM, Mike Latimer <mlatimer@xxxxxxxx> wrote:
> On Wednesday, January 07, 2015 09:38:31 AM Ian Campbell wrote:
>> That's exactly what I was about to suggest as I read the penultimate
>> paragraph, i.e. keep waiting so long as some reasonable delta occurs on
>> each iteration.
>
> Thanks, Ian.
>
> I wonder if there is a future-safe threshold on the amount of delta that
> indicates progress is being made. Should some minimum safe progress amount or
> percentage be set, or is it better to just make sure free memory is increasing
> at the end of each iteration of the loop?
>
> For example, the following simple change just tracks free_memkb and only
> decrements the retry count if it has not increased since the last check:
>
> ----------------------
> diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
> index ed0d478..4cf2991 100644
> --- a/tools/libxl/xl_cmdimpl.c
> +++ b/tools/libxl/xl_cmdimpl.c
> @@ -2196,7 +2196,7 @@ static int preserve_domain(uint32_t *r_domid,
> libxl_event *event,
>  static int freemem(uint32_t domid, libxl_domain_build_info *b_info)
>  {
>      int rc, retries = 3;
> -    uint32_t need_memkb, free_memkb;
> +    uint32_t need_memkb, free_memkb, free_memkb_prev = 0;
>
>      if (!autoballoon)
>          return 0;
> @@ -2229,7 +2229,10 @@ static int freemem(uint32_t domid,
> libxl_domain_build_info *b_info)
>          if (rc < 0)
>              return rc;
>
> -        retries--;
> +        /* only decrement retry count if free_memkb is not increasing */
> +        if (free_memkb <= free_memkb_prev)
> +            retries--;
> +        free_memkb_prev = free_memkb;

I would:
1. Reset the retries after a successful increase
2. Not allow free_memkb_prev to go down.

So maybe something like the following?

 if (free_memkb <= free_memkb_prev) {
   retries--;
 } else {
   retries = MAX_RETRIES;
   free_memkb_prev = free_memkb;
 }

I'm inclined to say we could add an option to say "wait forever", or
to increase the period of the checks; but ultimately at some point
someone (either xl or the human) needs to timeout and say, "This is
never going to finish".  10s seems like a very conservative default.

 -George

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