From: Ingo Molnar <mingo@kernel.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
live-patching@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>, Jiri Slaby <jslaby@suse.cz>,
"H. Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 4/8] objtool: add undwarf debuginfo generation
Date: Fri, 7 Jul 2017 11:44:37 +0200 [thread overview]
Message-ID: <20170707094437.2vgosia5hjg2wsut@gmail.com> (raw)
In-Reply-To: <20170706203636.lcwfjsphmy2q464v@treble>
* Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Thu, Jun 29, 2017 at 10:06:52AM -0500, Josh Poimboeuf wrote:
> > On Thu, Jun 29, 2017 at 04:46:18PM +0200, Ingo Molnar wrote:
> > >
> > > * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > >
> > > > > Plus, shouldn't we use __packed for 'struct undwarf' to minimize the
> > > > > structure's size (to 6 bytes AFAICS?) - or is optimal packing of the main
> > > > > undwarf array already guaranteed on every platform with this layout?
> > > >
> > > > Ah yes, it should definitely be packed (assuming that doesn't affect performance
> > > > negatively).
> > >
> > > So if I count that correctly that should shave another ~1MB off a typical ~4MB
> > > table size?
> >
> > Here's what my Fedora kernel looks like *before* the packed change:
> >
> > $ eu-readelf -S vmlinux |grep undwarf
> > [15] .undwarf_ip PROGBITS ffffffff81f776d0 011776d0 0012d9d0 0 A 0 0 1
> > [16] .undwarf PROGBITS ffffffff820a50a0 012a50a0 0025b3a0 0 A 0 0 1
> >
> > The total undwarf data size is ~3.5MB.
> >
> > There are 308852 entries of two parallel arrays:
> >
> > * .undwarf (8 bytes/entry) = 2470816 bytes
> > * .undwarf_ip (4 bytes/entry) = 1235408 bytes
> >
> > If we pack undwarf, reducing the size of the .undwarf entries by two
> > bytes, it will save 308852 * 2 = 617704.
> >
> > So the savings will be ~600k, and the typical size will be reduced to ~3MB.
>
> Just for the record, while packing the struct from 8 to 6 bytes did save 600k,
> it also made the unwinder ~7% slower. I think that's probably an ok tradeoff,
> so I'll leave it packed in v3.
So, out of curiosity, I'm wondering where that slowdown comes from: on modern x86
CPUs indexing by units of 6 bytes ought to be just as fast as indexing by 8 bytes,
unless I'm missing something? Is it maybe the not naturally aligned 32-bit words?
Or maybe there's some bad case of a 32-bit word crossing a 64-byte cache line
boundary that hits some pathological aspect of the CPU? We could probably get
around any such problems by padding by 2 bytes on 64-byte boundaries - that's only
a ~3% data size increase. The flip side would be a complication of the data
structure and its accessors - which might cost more in terms of code generation
efficiency than it buys us to begin with ...
Also, there's another aspect besides RAM footprint: a large data structure that is
~20% smaller means 20% less cache footprint: which for cache cold lookups might
matter more than the direct computational cost.
Thanks,
Ingo
next prev parent reply other threads:[~2017-07-07 9:44 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-28 15:11 [PATCH v2 0/8] x86: undwarf unwinder Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 1/8] objtool: move checking code to check.c Josh Poimboeuf
2017-06-30 13:12 ` [tip:core/objtool] objtool: Move " tip-bot for Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 2/8] objtool, x86: add several functions and files to the objtool whitelist Josh Poimboeuf
2017-06-30 13:12 ` [tip:core/objtool] objtool, x86: Add " tip-bot for Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 3/8] objtool: stack validation 2.0 Josh Poimboeuf
2017-06-30 8:32 ` Ingo Molnar
2017-06-30 13:23 ` Josh Poimboeuf
2017-06-30 13:26 ` Josh Poimboeuf
2017-06-30 14:09 ` [PATCH] objtool: silence warnings for functions which use iret Josh Poimboeuf
2017-06-30 17:49 ` [tip:core/objtool] objtool: Silence warnings for functions which use IRET tip-bot for Josh Poimboeuf
2017-06-30 13:13 ` [tip:core/objtool] objtool: Implement stack validation 2.0 tip-bot for Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 4/8] objtool: add undwarf debuginfo generation Josh Poimboeuf
2017-06-29 7:14 ` Ingo Molnar
2017-06-29 13:40 ` Josh Poimboeuf
2017-06-29 7:25 ` Ingo Molnar
2017-06-29 14:04 ` Josh Poimboeuf
2017-06-29 14:46 ` Ingo Molnar
2017-06-29 15:06 ` Josh Poimboeuf
2017-07-06 20:36 ` Josh Poimboeuf
2017-07-07 9:44 ` Ingo Molnar [this message]
2017-07-11 2:58 ` Josh Poimboeuf
2017-07-11 8:40 ` Ingo Molnar
2017-06-28 15:11 ` [PATCH v2 5/8] objtool, x86: add facility for asm code to provide unwind hints Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 6/8] x86/entry: add unwind hint annotations Josh Poimboeuf
2017-06-29 17:53 ` Josh Poimboeuf
2017-06-29 18:50 ` Andy Lutomirski
2017-06-29 19:05 ` Josh Poimboeuf
2017-06-29 21:09 ` Andy Lutomirski
2017-06-29 21:41 ` Josh Poimboeuf
2017-06-29 22:59 ` Andy Lutomirski
2017-06-30 2:12 ` Josh Poimboeuf
2017-06-30 5:05 ` Andy Lutomirski
2017-06-30 5:41 ` Andy Lutomirski
2017-06-30 13:11 ` Josh Poimboeuf
2017-06-30 15:44 ` Andy Lutomirski
2017-06-30 15:55 ` Josh Poimboeuf
2017-06-30 15:56 ` Andy Lutomirski
2017-06-30 16:16 ` Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 7/8] x86/asm: add unwind hint annotations to sync_core() Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 8/8] x86/unwind: add undwarf unwinder Josh Poimboeuf
2017-06-29 7:55 ` [PATCH v2 0/8] x86: " Ingo Molnar
2017-06-29 14:12 ` Josh Poimboeuf
2017-06-29 19:13 ` Josh Poimboeuf
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