From: Thomas Monjalon <thomas@monjalon.net>
To: "Morten Brørup" <mb@smartsharesystems.com>
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, 07 Jun 2023 10:10:20 +0200 [thread overview]
Message-ID: <3590085.WbyNdk4fJJ@thomas> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35D87995@smartserver.smartshare.dk>
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.
next prev parent reply other threads:[~2023-06-07 8:10 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 [this message]
2023-06-07 8:33 ` Morten Brørup
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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