lkml.org 
[lkml]   [2012]   [Feb]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups + docs

* H. Peter Anvin <hpa@zytor.com> wrote:

> On 02/21/2012 11:25 PM, Ingo Molnar wrote:
> >
> > There is a fundamental assymetry, and intentionally so. You
> > *really* have to think what the common case is, and make
> > sure the build defaults to that. It's not the end of the
> > world to have it flipped over, but there's costs and those
> > costs are higher even in the branch path than a regular
> > likely()/unlikely().
>
> No, not really -- it's still an unconditional branch, which
> means you will not tax the branch predictor in any way and
> which can be followed by the front end without taking a
> speculation hit. [...]

You are talking about CPU level costs, I am also talking about
costs introduced at build time.

Fact is, jump-label unlikely branches are moved *out of line*:
they are often in unlikely portions of the function (near other
unlikely branches), with instruction cache granularity costs and
potentially higher instruction-cache miss costs attached, etc.

You are missing three important aspects:

Firstly, instead of:

ins1
ins2
ins3
ins4
ins5
ins-compare
ins-branch
ins6
ins7
ins8
ins9
ins10

We have:

ins1
ins2
ins3
ins4
ins5
ins-jump

[ hole ]

ins6
ins7
ins8
ins9
ins10
ins-jump back

Where the 'hole' fragments the instruction cache layout. Given
that most of kernel execution is instruction-cache-cold, the
'straightness' of kernel code matters quite a bit.

Secondly, there's build time instruction scheduling costs as
well: GCC will prefer the likely branch over the unlikely one,
so we might see extra instructions in the out-of-line code:


ins1
ins2
ins3
ins4
ins5
ins-jump

[ hole ]

ins-extra-1
ins-extra-2
ins6
ins7
ins8
ins9
ins10
ins-jump back

In that sense jump labels are unlikely() branches combined with
a patching mechanism.

Thus *both* aspects are important: if a branch is *truly* 50/50
then it's quite possibly *NOT* a correct optimization to use
jump-labels as the 'uncommon' code goes through extra hoops and
fragments out of the fastpath, which in quite many real life
cases can outstrip the advantage of the avoidance of a single
branch ...

Thirdly,

even if it's a correct optimization and both branches happen to
outperform the pre-jump-label version, regardless of the
direction of the jump label flag, it's *STILL* fundamentally
assymetric: due to the hole and due to the possible extra
instructions the out of line code will be slower by a few
instruction and the NOP fall-through will be faster.

This is fundamentally so, and any naming that tries to *hide*
that assymetry and the associated micro-costs is confused.

Thanks,

Ingo


\
 
 \ /
  Last update: 2012-02-22 08:51    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site