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Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions |
Date: |
Wed, 09 Jul 2014 10:19:51 +0100 |
Al Viro writes:
> On Tue, Jul 08, 2014 at 08:32:55PM +0100, Peter Maydell wrote:
> On 8 July 2014 18:20, Al Viro <address@hidden> wrote:
>> > On Tue, Jul 08, 2014 at 05:33:16PM +0100, Peter Maydell wrote:
<snip>
> Again, gprof isn't particulary useful - kernel-side profilers are at least as
> good. So I suspect that most of the people running into that simply shrug and
> use those instead. Narrowing it down to -pie didn't take long and I can
> confirm that this is the root cause of that breakage. Should make debugging
> said toolchain bug a bit easier, if anybody cares to do that...
>
>> > Stats I quoted were from qemu-system-alpha booting debian/lenny (5.10) and
>> > going through their kernel package build. I have perf report in front of
>> > me right now; the top ones are
>> > 41.77% qemu-system-alp perf-24701.map [.] 0x7fbbee558930
>> > 11.78% qemu-system-alp qemu-system-alpha [.] cpu_alpha_exec
>>
>> > and cpu_alpha_exec() spends most of the time in inlined tb_find_fast().
>> > It might be worth checking the actual distribution of the hash of virt
>> > address used by that sucker - I wonder if dividing its argument by 4
>> > wouldn't improve the things, but I don't have stats on actual frequency
>> > of conflicts, etc. In any case, the first lump (42%) seems to be tastier
>> > ;-)
>>
>> Depends on your point of view -- arguably we ought to be spending *more*
>> time executing translated guest code... (As you say, the problem is that
>> we don't have any breakdown of what things might turn out to be hotspots
>> in the translated code.)
>
> Might be a fun project to teach perf that hits in such-and-such page should
> lead to lookup in a table annotating it. As in "offsets 42..69 should be
> recorded as (<this address> + offset - 42). Then tcg could generate
> such tables and we'd get information like "that much time is spent in
> the second host insn of instances of that code pattern generated by
> tcg_gen_shr_i64", etc.
>
> No idea if anything of that sort exists - qemu is not the only possible user
> for that; looks like it might be useful for any JIT profiling, so somebody
> could've done that already...
Handily our patch tracker has remembered what I couldn't find ;-)
https://patches.linaro.org/27229/
As I mentioned previously I plan to clean these up over the next week.
--
Alex Bennée
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, (continued)
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Peter Maydell, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Peter Maydell, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Peter Maydell, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/10
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions,
Alex Bennée <=
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Alex Bennée, 2014/07/09
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/08
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/09
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/09
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/10
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/09