From: Steven Rostedt <rostedt@goodmis.org>
To: Pekka Enberg <penberg@kernel.org>
Cc: Minchan Kim <minchan.kim@gmail.com>,
Theodore Ts'o <tytso@mit.edu>,
Christoph Lameter <cl@linux-foundation.org>,
Pekka Enberg <penberg@cs.helsinki.fi>,
Matt Mackall <mpm@selenic.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
David Rientjes <rientjes@google.com>,
npiggin@kernel.dk
Subject: Re: Should we be using unlikely() around tests of GFP_ZERO?
Date: Mon, 03 Jan 2011 08:45:51 -0500 [thread overview]
Message-ID: <1294062351.3948.7.camel@gandalf.stny.rr.com> (raw)
In-Reply-To: <AANLkTinz52Ky5BhU-gHq8vx9=1uoN+iuDn1f0C8fnSjQ@mail.gmail.com>
On Mon, 2011-01-03 at 09:40 +0200, Pekka Enberg wrote:
> Hi,
>
> On Mon, Jan 3, 2011 at 8:48 AM, Theodore Ts'o <tytso@mit.edu> wrote:
> >> Given the patches being busily submitted by trivial patch submitters to
> >> make use kmem_cache_zalloc(), et. al, I believe we should remove the
> >> unlikely() tests around the (gfp_flags & __GFP_ZERO) tests, such as:
> >>
> >> - if (unlikely((flags & __GFP_ZERO) && objp))
> >> + if ((flags & __GFP_ZERO) && objp)
> >> memset(objp, 0, obj_size(cachep));
> >>
> >> Agreed? If so, I'll send a patch...
>
> On Mon, Jan 3, 2011 at 5:46 AM, Minchan Kim <minchan.kim@gmail.com> wrote:
> > I support it.
>
> I guess the rationale here is that if you're going to take the hit of
> memset() you can take the hit of unlikely() as well. We're optimizing
> for hot call-sites that allocate a small amount of memory and
> initialize everything themselves. That said, I don't think the
> unlikely() annotation matters much either way and am for removing it
> unless people object to that.
>
> On Mon, Jan 3, 2011 at 5:46 AM, Minchan Kim <minchan.kim@gmail.com> wrote:
> > Recently Steven tried to gather the information.
> > http://thread.gmane.org/gmane.linux.kernel/1072767
> > Maybe he might have a number for that.
>
> That would be interesting, sure.
Note, you could do it yourself too. Just enable:
Kernel Hacking -> Tracers -> Branch Profiling
(Trace likely/unlikely profiler)
CONFIG_PROFILE_ANNOTATED_BRANCHES
Then search /debug/tracing/trace_stats/branch_annotated.
(hmm, the help in Kconfig is wrong, I need to fix that)
Anyway, here's my box. I just started it an hour ago, and have not been
doing too much on it yet. But here's what I got (using SLUB)
correct incorrect % Function File Line
------- --------- - -------- ---- ----
6890998 2784830 28 slab_alloc slub.c 1719
That's incorrect 28% of the time.
-- Steve
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next prev parent reply other threads:[~2011-01-03 13:45 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-01-02 23:48 Theodore Ts'o
2011-01-03 3:46 ` Minchan Kim
2011-01-03 7:40 ` Pekka Enberg
2011-01-03 13:45 ` Steven Rostedt [this message]
2011-01-03 14:10 ` Pekka Enberg
2011-01-03 14:26 ` Steven Rostedt
2011-01-03 13:58 ` Ted Ts'o
2011-01-03 14:09 ` Pekka Enberg
2011-01-03 17:23 ` Matt Mackall
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