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Date: Fri, 06 Sep 2013 12:14:27 -0700
From: Alfred Perlstein <bright@mu.org>
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To: hiren panchasara <hiren.panchasara@gmail.com>
Subject: Re: mbuf autotuning changes
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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.

-Alfred

-- 
Alfred Perlstein