hot path optimizations in uma_zalloc() & uma_zfree()

[Max Laier]

On Thursday 30 June 2005 00:08, ant wrote:
> I just tryed to make buckets management in perCPU cache like in
> Solaris (see paper of Jeff Bonwick - Magazines and Vmem)
> and got perfomance gain around 10% in my test program.
> Then i made another minor code optimization and got another 10%.
> The program just creates and destroys sockets in loop.
>
> I suppose the reason of first gain lies in increasing of cpu cache hits.
> In current fbsd code allocations and freeings deal with
> separate buckets. Buckets are changed when one of them
> became full or empty first. In Solaris this work is pure LIFO:
> i.e. alloc() and free() work with one bucket - the current bucket
> (it is called magazine there), that's why cache hit rate is bigger.
>
> Another optimization is very trivial, for example:
> -   bucket->ub_cnt--;
> -   item = bucket->ub_bucket[bucket->ub_cnt];
> +   item = bucket->ub_bucket[--bucket->ub_cnt];
> (see the patch)

Might be me, but this doesn't change the generated object code at all (modulo 
the changed __line__ in debugging).

> the patch is for uma_core.c from RELENG_5, but i checked
> uma_core.c in CURRENT - it's the same regarding to thiese
> improvements. I don't have any commit rights, so the patch
> is just for reviewing. Here it is:
>
> --- sys/vm/uma_core.c.orig Wed Jun 29 21:46:52 2005
> +++ sys/vm/uma_core.c Wed Jun 29 23:09:32 2005
> @@ -1830,8 +1830,7 @@
>
>   if (bucket) {
>    if (bucket->ub_cnt > 0) {
> -   bucket->ub_cnt--;
> -   item = bucket->ub_bucket[bucket->ub_cnt];
> +   item = bucket->ub_bucket[--bucket->ub_cnt];
>  #ifdef INVARIANTS
>     bucket->ub_bucket[bucket->ub_cnt] = NULL;
>  #endif

Okay, but no effect according to my reading of the object code.

> @@ -2263,8 +2262,7 @@
>    if (bucket->ub_cnt < bucket->ub_entries) {
>     KASSERT(bucket->ub_bucket[bucket->ub_cnt] == NULL,
>         ("uma_zfree: Freeing to non free bucket index."));
> -   bucket->ub_bucket[bucket->ub_cnt] = item;
> -   bucket->ub_cnt++;
> +   bucket->ub_bucket[bucket->ub_cnt++] = item;

This changes semantics, as far as I understand.  Might be a consequence of the 
other work you are doing, but doesn't seem right from a first glance.

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