public inbox for gcc-patches@gcc.gnu.org
 help / color / mirror / Atom feed
From: "Martin Liška" <mliska@suse.cz>
To: Richard Biener <richard.guenther@gmail.com>,
	Jakub Jelinek <jakub@redhat.com>,
	Andi Kleen <andi@firstfloor.org>
Cc: Jeff Law <law@redhat.com>, Nathan Sidwell <nathan@acm.org>,
	gcc-patches@gcc.gnu.org, jh@suse.cz
Subject: Re: [PATCH] Set -fprofile-update=atomic when -pthread is present
Date: Wed, 07 Sep 2016 11:41:00 -0000	[thread overview]
Message-ID: <5798a459-2fc7-d82a-f89b-30a45a03c831@suse.cz> (raw)
In-Reply-To: <EE9C60C5-B43D-403B-9066-A44DADF5B4F3@gmail.com>

On 08/18/2016 06:06 PM, Richard Biener wrote:
> On August 18, 2016 5:54:49 PM GMT+02:00, Jakub Jelinek <jakub@redhat.com> wrote:
>> On Thu, Aug 18, 2016 at 08:51:31AM -0700, Andi Kleen wrote:
>>>> I'd prefer to make updates atomic in multi-threaded applications.
>>>> The best proxy we have for that is -pthread.
>>>>
>>>> Is it slower, most definitely, but odds are we're giving folks
>>>> garbage data otherwise, which in many ways is even worse.
>>>
>>> It will likely be catastrophically slower in some cases. 
>>>
>>> Catastrophically as in too slow to be usable.
>>>
>>> An atomic instruction is a lot more expensive than a single
>> increment. Also
>>> they sometimes are really slow depending on the state of the machine.
>>
>> Can't we just have thread-local copies of all the counters (perhaps
>> using
>> __thread pointer as base) and just atomically merge at thread
>> termination?
> 
> I suggested that as well but of course it'll have its own class of issues (short lived threads, so we need to somehow re-use counters from terminated threads, large number of threads and thus using too much memory for the counters)
> 
> Richard.

Hello.

I've got written the approach on my TODO list, let's see whether it would be doable in a reasonable amount of time.

I've just finished some measurements to illustrate slow-down of -fprofile-update=atomic approach.
All numbers are: no profile, -fprofile-generate, -fprofile-generate -fprofile-update=atomic
c-ray benchmark (utilizing 8 threads, -O3): 1.7, 15.5., 38.1s
unrar (utilizing 8 threads, -O3): 3.6, 11.6, 38s
tramp3d (1 thread, -O3): 18.0, 46.6, 168s

So the slow-down is roughly 300% compared to -fprofile-generate. I'm not having much experience with default option
selection, but these numbers can probably help.

Thoughts?
Martin

> 
>> 	Jakub
> 
> 

  reply	other threads:[~2016-09-07 11:37 UTC|newest]

Thread overview: 95+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-08-01  8:50 [PATCH 0/4] Various GCOV/PGO improvements marxin
2016-08-01  8:49 ` [PATCH 3/4] Fix typo in gcov.texi marxin
2016-08-01  8:50 ` [PATCH 4/4] Add tests for __gcov_dump and __gcov_reset marxin
2016-08-01  8:50 ` [PATCH 2/4] Remove __gcov_indirect_call_profiler marxin
2016-08-01  8:50 ` [PATCH 1/4] Cherry-pick fprofile-generate-atomic from google/gcc-4_9 branch marxin
2016-08-01 12:22   ` Nathan Sidwell
2016-08-01 13:29     ` Martin Liška
2016-08-04 14:48       ` Nathan Sidwell
2016-08-04 15:34         ` Martin Liška
2016-08-04 16:43           ` Nathan Sidwell
2016-08-04 17:03             ` Nathan Sidwell
2016-08-05  8:55               ` Martin Liška
2016-08-05 12:38                 ` Nathan Sidwell
2016-08-05 12:48                   ` Martin Liška
2016-08-05 13:14                     ` Nathan Sidwell
2016-08-05 13:43                       ` Martin Liška
2016-08-08 13:59                         ` [PATCH 5/N] Add new *_atomic counter update function, (-fprofile-update=atomic) Martin Liška
2016-08-08 15:24                           ` Nathan Sidwell
2016-08-08 16:51                             ` Martin Liška
2016-08-08 17:03                               ` Martin Liška
2016-08-09 12:36                                 ` Nathan Sidwell
2016-08-08 16:56                             ` [PATCH] Fix POW2 histogram Martin Liška
2016-08-09  8:41                               ` [PATCH 2/N] Fix usage of " Martin Liška
2016-08-09 12:37                                 ` Nathan Sidwell
2016-08-09 12:34                               ` [PATCH] Fix " Nathan Sidwell
2016-08-09 11:24                         ` [PATCH] Set -fprofile-update=atomic when -pthread is present Martin Liška
2016-08-09 12:40                           ` Nathan Sidwell
2016-08-09 19:04                           ` Andi Kleen
2016-08-12 13:31                             ` Martin Liška
2016-08-18  3:16                               ` Jeff Law
2016-08-18 11:02                                 ` Nathan Sidwell
2016-08-18 15:51                                 ` Andi Kleen
2016-08-18 15:53                                   ` Jeff Law
2016-10-03 12:13                                     ` Martin Liška
2016-10-03 12:26                                       ` Nathan Sidwell
2016-10-03 16:46                                         ` Jeff Law
2016-10-03 17:52                                           ` Andi Kleen
2016-10-04 12:05                                         ` Martin Liška
2016-10-05 17:54                                           ` Jeff Law
2016-10-13 15:34                                           ` [PATCH] Introduce -fprofile-update=maybe-atomic Martin Liška
2016-10-31  9:13                                             ` Martin Liška
2016-11-10 13:19                                               ` Martin Liška
2016-11-10 15:43                                                 ` Nathan Sidwell
2016-11-10 15:55                                                   ` David Edelsohn
2016-11-10 16:18                                                     ` Nathan Sidwell
2016-11-10 15:58                                                   ` Martin Liška
2016-11-10 16:17                                                     ` David Edelsohn
2016-11-10 16:24                                                       ` Martin Liška
2016-11-10 17:31                                                         ` Nathan Sidwell
2016-11-11 10:48                                                           ` Martin Liška
2016-11-11 15:11                                                             ` Nathan Sidwell
2016-11-10 16:14                                                   ` Nathan Sidwell
2016-11-10 16:16                                                     ` David Edelsohn
2016-08-18 15:54                                   ` [PATCH] Set -fprofile-update=atomic when -pthread is present Jakub Jelinek
2016-08-18 16:06                                     ` Richard Biener
2016-09-07 11:41                                       ` Martin Liška [this message]
     [not found]                                         ` <CAFiYyc0UaSzXhZmyG9QRkHGT4JFowxBfE2yb-NvXE=hR1xafdA@mail.gmail.com>
2016-09-15 10:18                                           ` [RFC] Speed-up -fprofile-update=atomic Martin Liška
2016-10-04  9:45                                             ` Richard Biener
2016-10-12 13:53                                               ` Martin Liška
2016-10-13  9:43                                                 ` Richard Biener
2016-10-17 11:47                                                   ` Martin Liška
     [not found]                                                     ` <CAFiYyc3eDT4g926PPZuktz5fEW=k-PibAcxhigx4GBcxoXNJFQ@mail.gmail.com>
2016-10-24 12:09                                                       ` Martin Liška
     [not found]                                                         ` <CAFiYyc1tSdTdqqkHcMp+dgE43+8tHL6kY8E07TCHoZBeUT-ggQ@mail.gmail.com>
2016-10-25 14:32                                                           ` Martin Liška
2016-10-26  9:29                                                             ` Richard Biener
2016-10-26  9:32                                                               ` Richard Biener
2016-08-18 16:04                                   ` [PATCH] Set -fprofile-update=atomic when -pthread is present Richard Biener
2016-08-10 12:57                         ` [PATCH 1/4] Cherry-pick fprofile-generate-atomic from google/gcc-4_9 branch Nathan Sidwell
2016-08-13 12:14                         ` [BUILDROBOT] avr broken (was: [PATCH 1/4] Cherry-pick fprofile-generate-atomic from google/gcc-4_9 branch) Jan-Benedict Glaw
     [not found]                           ` <4455937b-eba7-fe66-fe1a-3172567dd1e4@suse.cz>
2016-08-16 13:36                             ` [BUILDROBOT] avr broken Nathan Sidwell
     [not found]                               ` <617e8799-b7db-fefd-b3a3-842e9a7decfd@suse.cz>
2016-08-16 14:31                                 ` Nathan Sidwell
2016-08-16 17:05                                   ` Jan-Benedict Glaw
2016-08-16 18:26                                     ` Nathan Sidwell
2016-08-17  7:21                                       ` Denis Chertykov
2016-08-17  7:22                                         ` Martin Liška
2016-08-17  8:11                                       ` Jan-Benedict Glaw
2016-08-16 12:56                         ` [PATCH] Detect whether target can use -fprofile-update=atomic Martin Liška
2016-08-16 14:31                           ` Nathan Sidwell
2016-09-06 10:57                             ` Martin Liška
2016-09-06 11:17                               ` David Edelsohn
2016-09-06 12:15                                 ` Nathan Sidwell
2016-09-06 12:39                                   ` Jakub Jelinek
2016-09-06 12:43                                     ` David Edelsohn
2016-09-06 12:41                                   ` David Edelsohn
2016-09-06 12:51                                     ` Martin Liška
2016-09-06 13:13                                       ` Jakub Jelinek
2016-09-06 13:15                                         ` Martin Liška
2016-09-06 13:45                                           ` Jakub Jelinek
2016-09-06 13:50                                             ` Martin Liška
2016-09-06 14:06                                               ` Jakub Jelinek
2016-09-07  7:52                                               ` Christophe Lyon
2016-09-07  9:35                                                 ` Martin Liška
2016-09-07 16:06                                                   ` Christophe Lyon
2016-09-12 20:20                                                   ` Jeff Law
2016-09-29  8:31                                               ` Rainer Orth
2016-08-01 12:11 ` [PATCH 0/4] Various GCOV/PGO improvements Nathan Sidwell

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=5798a459-2fc7-d82a-f89b-30a45a03c831@suse.cz \
    --to=mliska@suse.cz \
    --cc=andi@firstfloor.org \
    --cc=gcc-patches@gcc.gnu.org \
    --cc=jakub@redhat.com \
    --cc=jh@suse.cz \
    --cc=law@redhat.com \
    --cc=nathan@acm.org \
    --cc=richard.guenther@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).