mbuf autotuning changes

[Alfred Perlstein]

On 9/6/13 12:36 PM, hiren panchasara wrote:
> On Fri, Sep 6, 2013 at 12:14 PM, Alfred Perlstein <bright at mu.org> wrote:
>
>> On 9/6/13 12:10 PM, hiren panchasara wrote:
>>
>>> tunable_mbinit() in kern_mbuf.c looks like this:
>>>
>>> 119         /*
>>> 120          * The default limit for all mbuf related memory is 1/2 of all
>>> 121          * available kernel memory (physical or kmem).
>>> 122          * At most it can be 3/4 of available kernel memory.
>>> 123          */
>>> 124         realmem = qmin((quad_t)physmem * PAGE_SIZE,
>>> 125             vm_map_max(kmem_map) - vm_map_min(kmem_map));
>>> 126         maxmbufmem = realmem / 2;
>>> 127         TUNABLE_QUAD_FETCH("kern.ipc.**maxmbufmem", &maxmbufmem);
>>> 128         if (maxmbufmem > realmem / 4 * 3)
>>> 129                 maxmbufmem = realmem / 4 * 3;
>>>
>>> If I am reading the code correctly, we loose the value on line 126 when we
>>> do FETCH on line 127.
>>>
>>> And after line 127, if we havent specified kern.ipc.maxmbufmem (in
>>> loader.conf - I guess...), we set that value to 0.
>>>
>>> And because of that the if condition on line 128 is almost always false?
>>>
>>> What am I missing here?
>>>
>>> Thanks,
>>> Hiren
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>>>
>>>   I think TUNABLE_*_FETCH will only write to the variable if it explicitly
>> set.
>>
>> Meaning, unless the user actually sets a value in loader.conf then 127 is
>> a no-op.
>>
> Thanks Navdeep and Alfred.
>
> Thats correct. Its not touching the var if its not set.
>
> I guess the other TUNABLE_INT_FETCHs later in the function checking for
> variable ==0 confused me. i.e. nmbclusters.
>
> 131         TUNABLE_INT_FETCH("kern.ipc.nmbclusters", &nmbclusters);
> 132         if (nmbclusters == 0)
> 133                 nmbclusters = maxmbufmem / MCLBYTES / 4;
>
> But those are global variable so here we are just checking if they are
> explicitly set of not. If not, we will set them.
>
> For maxmbufmem, we will set it to 1/2 the realmem. and if user sets it
> explicitly than we will make sure its not more than 3/4 of the realmem.
Yes.  It's somewhat confusing.

I'm all for adding comments to this effect if you have the time and 
inclination.

-Alfred