Memory allocation performance

Alexander Motin mav at
Sat Feb 2 01:31:41 PST 2008

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

Alexander Motin

More information about the freebsd-performance mailing list