From: Tom Kacvinsky <tkacvins@gmail.com>
To: Binutils <binutils@sourceware.org>
Subject: Re: Question about readelf output from shared library built with lld, gold, and bfd linkers
Date: Mon, 21 Nov 2022 09:51:20 -0500 [thread overview]
Message-ID: <CAG_eJLdvkmQm6qLOZst4RLVcKTLDPpo0aZ4FDz6CQtiO0nFtWQ@mail.gmail.com> (raw)
In-Reply-To: <e3fc471d-ec8c-c483-11a7-3f291ffa89ce@redhat.com>
[-- Attachment #1: Type: text/plain, Size: 2195 bytes --]
On Mon, Nov 21, 2022 at 9:14 AM Nick Clifton <nickc@redhat.com> wrote:
> Hi Tom,
>
> > Isee the following output from readelf for the lld, gold and bfd linkers
> > (binutils 2.39, lld 14.0.6)
>
> To be clear - this is not a readelf problem. It is showing you the correct
> results. It is the fact that the three linkers are not producing identical
> output and instead showing slight variations in their layout of the linked
> binary that is bothering you, yes ?
>
It's not bothering me so much as I was wondering if this would point to an
issue with slow start up times. Reading your reply in totality, I now see
this
should not make a difference.
> In general variations in the layout like this should not make any
> difference
> to the program#s startup time. There might - possibly - be variations in
> performance due to affects like cache misses and the like, but this is hard
> to quantify in isolation.
>
I didn't stop to think about caching and the like for relocations at start
up. I was
testing with the perf tool and LD_DEBUG=statistics to see how much time was
spent in start up by symbols/relocations. Results varied from run to run,
so I can
now see how caching may play into this.
> Output below. Notice how the ordering is different in each case. The
> > interesting thing about this is, and why I am looking at various
> > differences between object code linked by these three linkers, is that I
> am
> > trying to track down why startup times are slower due to relocations
> (based
> > on the perf tool output). Would any of these differences make, well, a
> > difference in startup time?
>
> I don't think so. The number of relocations is the same, so the amount
> of start up time spent resolving them should effectively be the same as
> well.
>
> I assume that you have compared started up times when linking with "-z now"
> vs "-z lazy" ?
>
I thought -z lazy was the default, and that if you wanted the equivalent of
LD_BIND_NOW=1
on the command line, then one would use -z now. We don't want the latter,
we already spawn
enough processes that -z now would slow things down even more. I can try
with -z lazy and
report back.
Thanks for the input,
Tom
next prev parent reply other threads:[~2022-11-21 14:51 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-20 16:37 Tom Kacvinsky
2022-11-21 14:14 ` Nick Clifton
2022-11-21 14:51 ` Tom Kacvinsky [this message]
2022-11-21 15:16 ` Nick Clifton
2022-11-21 15:33 ` Tom Kacvinsky
2022-11-21 16:48 ` Nick Clifton
2022-11-21 17:43 ` Tom Kacvinsky
2022-11-22 11:20 ` Nick Clifton
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