Re: xl mem-max error

[Zhigang Wang <zhigang.x.wang@oracle.com>]

On 11/12/2014 07:03 AM, Ian Campbell wrote:
> On Mon, 2014-11-10 at 10:38 -0500, Zhigang Wang wrote:
>> OK. Let me try my best:
>>
>>>>> I'm confused by the description of what's going on, in particular the
>>>>> mixing of mem-max commands and target xenstore nodes (since the former
>>>>> doesn't really affect the latter).
>>>>>
>>>>> How was the domain started (memory= and maxmem=).
>>
>> xl create with 'memory = 700', no maxmem been set. I think it means maxmem = memory for this case.
>>
>>>>> What were static-max and target at the point?
>>
>>      /local/domain/3/memory/static-max = "716800"
>>      /local/domain/3/memory/target = "716801"
>>
>>>>> What did they change to when xl mem-max was issued? 
>>
>> When I issue 'xl mem-max <domid> 700', static-max and target in xenstore will not change, but it will cause the command to fail.
>>
>> Because: "libxl: error: libxl.c:4549:libxl_domain_setmaxmem: memory_static_max must be greater than or or equal to memory_dynamic_max"
>>
>>>>> What did you expect them to change to instead?
>>
>> I expect I can set the maxmem to the same value I initially set (700).
> 
> OK, thanks, got it. I think the use of xl mem-max is a bit of a
> red-herring, the issue here is that static-max < target at start of day.
> 
> I suspect there is either a rounding error somewhere or because of
> LIBXL_MAXMEM_CONSTANT being inconsistently applied to the two values
> somewhere along the line.
> 
> We had been planning[0] to remove this early in the 4.5 cycle, but as
> ever it never floated to the top of anyone's list. For 4.5 we should
> probably look at applying this fudge more consistently.
> 
> Ian.
> 
> [0] http://bugs.xenproject.org/xen/bug/23

Here is more info (correct me if something wrong):

1. libxl_types.idl:

    ("video_memkb", MemKB)

    MemKB = UInt(64, init_val = "LIBXL_MEMKB_DEFAULT", json_gen_fn = "libxl__uint64_gen_json")

  libxl.h: #define LIBXL_MEMKB_DEFAULT ~0ULL

  So video_memkb = -1 by default.

2. For hvm, we will set it into: 0, 16M, 8M according to hvm.vga.kind (libxl_create.c)

3. For pv guest, we will use default (-1).

4. When we calculate static-max and target memory:

    ents[0] = "memory/static-max";
    ents[1] = GCSPRINTF("%"PRId64, info->max_memkb);
    ents[2] = "memory/target";
    ents[3] = GCSPRINTF("%"PRId64, info->target_memkb - info->video_memkb);

  So target = static-max - (-1):
   
      /local/domain/3/memory/static-max = "716800"
      /local/domain/3/memory/target = "716801"

  Maybe this is the root cause why we include LIBXL_MAXMEM_CONSTANT.

Potential solutions:

1. Set b_info->video_memkb = 0; for pv guest.
2. Set LIBXL_MEMKB_DEFAULT = 0 (instead of -1): this will affect a lot things.
3. Also add LIBXL_MAXMEM_CONSTANT before doing comparison. (we should avoid this as we want to remove LIBXL_MAXMEM_CONSTANT)

Thanks,

Zhigang