Memory allocation performance

Robert Watson rwatson at
Sat Feb 2 01:59:45 PST 2008

On Sat, 2 Feb 2008, Alexander Motin wrote:

> Robert Watson wrote:
>> I guess the question is: where are the cycles going?  Are we suffering 
>> excessive cache misses in managing the slabs?  Are you effectively "cycling 
>> through" objects rather than using a smaller set that fits better in the 
>> cache?
> In my test setup only several objects from zone usually allocated same time, 
> but they allocated two times per every packet.
> To check UMA dependency I have made a trivial one-element cache which in my 
> test case allows to avoid two for four allocations per packet.

Avoiding unnecessary allocations is a good general principle, but duplicating 
cache logic is a bad idea.  If you're able to structure the below without 
using locking, it strikes me you'd do much better, especially if it's in a 
single processing pass.  Can you not use a per-thread/stack/session variable 
to avoid that?

> .....alloc.....
> -       item = uma_zalloc(ng_qzone, wait | M_ZERO);
> +       mtx_lock_spin(&itemcachemtx);
> +       item = itemcache;
> +       itemcache = NULL;
> +       mtx_unlock_spin(&itemcachemtx);

Why are you using spin locks?  They are quite a bit more expensive on several 
hardwawre platforms, and any environment it's safe to call uma_zalloc() from 
will be equally safe to use regular mutexes from (i.e., mutex-sleepable).

> +       if (item == NULL)
> +               item = uma_zalloc(ng_qzone, wait | M_ZERO);
> +       else
> +               bzero(item, sizeof(*item));
> -       uma_zfree(ng_qzone, item);
> +       mtx_lock_spin(&itemcachemtx);
> +       if (itemcache == NULL) {
> +               itemcache = item;
> +               item = NULL;
> +       }
> +       mtx_unlock_spin(&itemcachemtx);
> +       if (item)
> +               uma_zfree(ng_qzone, item);
> ...............
> To be sure that test system is CPU-bound I have throttled it with sysctl to 
> 1044MHz. With this patch my test PPPoE-to-PPPoE router throughput has grown 
> from 17 to 21Mbytes/s. Profiling results I have sent promised close results.
>> Is some bit of debugging enabled that shouldn't be, perhaps due to a 
>> failure of ifdefs?
> I have commented out all INVARIANTS and WITNESS options from GENERIC kernel 
> config. What else should I check?

Hence my request for drilling down a bit on profiling -- the question I'm 
asking is whether profiling shows things running or taking time that shouldn't 

Robert N M Watson
Computer Laboratory
University of Cambridge

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