From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Thomas Monjalon" <thomas@monjalon.net>
Cc: "Bruce Richardson" <bruce.richardson@intel.com>, <dev@dpdk.org>,
<olivier.matz@6wind.com>, <andrew.rybchenko@oktetlabs.ru>
Subject: RE: [PATCH] mempool: optimize get objects with constant n
Date: Wed, 7 Jun 2023 10:33:49 +0200 [thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35D87996@smartserver.smartshare.dk> (raw)
In-Reply-To: <3590085.WbyNdk4fJJ@thomas>
> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Wednesday, 7 June 2023 10.10
>
> 07/06/2023 10:03, Morten Brørup:
> > > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > Sent: Wednesday, 7 June 2023 09.52
> > >
> > > 18/04/2023 14:55, Bruce Richardson:
> > > > On Tue, Apr 18, 2023 at 01:29:49PM +0200, Morten Brørup wrote:
> > > > > From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> > > > > > On Tue, Apr 11, 2023 at 08:48:45AM +0200, Morten Brørup wrote:
> > > > > > > + if (__extension__(__builtin_constant_p(n)) && n <= cache->len) {
> > > > > > > + /*
> > > > > > > + * The request size is known at build time, and
> > > > > > > + * the entire request can be satisfied from the cache,
> > > > > > > + * so let the compiler unroll the fixed length copy loop.
> > > > > > > + */
> > > > > > > + cache->len -= n;
> > > > > > > + for (index = 0; index < n; index++)
> > > > > > > + *obj_table++ = *--cache_objs;
> > > > > > > +
> > > > > >
> > > > > > This loop looks a little awkward to me. Would it be clearer (and
> perhaps
> > > > > > easier for compilers to unroll efficiently if it was rewritten as:
> > > > > >
> > > > > > cache->len -= n;
> > > > > > cache_objs = &cache->objs[cache->len];
> > > > > > for (index = 0; index < n; index++)
> > > > > > obj_table[index] = cache_objs[index];
> > > > >
> > > > > The mempool cache is a stack, so the copy loop needs get the objects
> in
> > > decrementing order. I.e. the source index decrements and the destination
> index
> > > increments.
> > > > >
> > > >
> > > > BTW: Please add this as a comment in the code too, above the loop to
> avoid
> > > > future developers (or even future me), asking this question again!
> > >
> > > Looks like this request was missed.
> >
> > I intentionally omitted it, because I disagree with the suggestion.
> >
> > Otherwise, reminders that the mempool cache is a stack should be plastered
> all over the source code, not just here. For reference, this copy loop
> (without such a reminder) also exists elsewhere in the same file.
> >
> > Apologies for not responding to Bruce's request earlier.
>
> What about doing a general comment at the top of the function,
> with the assignment of the pointer at the end of the array:
>
> /* The cache is a stack, so copy will be in reverse order. */
> cache_objs = &cache->objs[cache->len];
>
> I could do it on apply if there is an agreement.
>
ACK from me.
For consistency, please also add the reminder to the two existing reverse order copy loops in rte_mempool_do_generic_get().
next prev parent reply other threads:[~2023-06-07 8:33 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-11 6:48 Morten Brørup
2023-04-18 11:06 ` Bruce Richardson
2023-04-18 11:29 ` Morten Brørup
2023-04-18 12:54 ` Bruce Richardson
2023-04-18 12:55 ` Bruce Richardson
2023-06-07 7:51 ` Thomas Monjalon
2023-06-07 8:03 ` Morten Brørup
2023-06-07 8:10 ` Thomas Monjalon
2023-06-07 8:33 ` Morten Brørup [this message]
2023-06-07 8:41 ` Morten Brørup
2023-04-18 15:15 ` Tyler Retzlaff
2023-04-18 15:30 ` Morten Brørup
2023-04-18 15:44 ` Tyler Retzlaff
2023-04-18 15:50 ` Morten Brørup
2023-04-18 16:01 ` Tyler Retzlaff
2023-04-18 16:05 ` Morten Brørup
2023-04-18 19:51 ` [PATCH v3] " Morten Brørup
2023-04-18 20:09 ` [PATCH v4] " Morten Brørup
2023-06-07 9:12 ` Thomas Monjalon
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