From: Thiago Macieira <thiago.macieira@intel.com>
To: Ville Voutilainen <ville.voutilainen@gmail.com>
Cc: Jonathan Wakely <jwakely.gcc@gmail.com>,
Thomas Rodgers <trodgers@redhat.com>,
libstdc++ <libstdc++@gcc.gnu.org>
Subject: Re: C++2a synchronisation inefficient in GCC 11
Date: Fri, 26 Feb 2021 16:53:13 -0800 [thread overview]
Message-ID: <4094126.biHxQJKGqB@tjmaciei-mobl1> (raw)
In-Reply-To: <CAFk2RUa8iCZYQ-b8Pv9yuBfOSikt08rMXcA1MvRQjtX=GO2UcA@mail.gmail.com>
On Friday, 26 February 2021 16:44:44 PST Ville Voutilainen wrote:
> On Sat, 27 Feb 2021 at 02:36, Thiago Macieira <thiago.macieira@intel.com>
wrote:
> > The danger is not how they're implemented today. The danger is the change.
> > That would imply a pre-change wait() is never woken up by a post-change
> > notify() or vice-versa. That in turn causes deadlocks in latches, barriers
> > and semaphores.
>
> Then we must be talking about something else than the topic of this
> thread, which says "inefficient". :P
Well, if they were efficient in the first place, we wouldn't need a change. :)
The problem is that they ARE inefficient and they're implemented in such a way
that a change to make them efficient breaks compatibility with the pre-change
implementation.
> I need to re-read your patches, but not until tomorrow.
Fair enough. I'm disconnecting from the work VPN right now and won't be
getting replies. Let me know if I can explain anything, though.
> Well, we want *bold* users to adopt new facilities early. Not all users.
> Feature-testing macros don't cover the problem of innocent users
> accidentally opting in to something that's in flux; the general expectation
> is that innocent users shouldn't do that.
I totally agree.
But how do we allow bold users to adopt the facilities in such a way not to
endanger the other (italic?) users?
> > > Please explain how your concerns about atomic waits
> > > are *anywhere near* the cost of doing that?
> >
> > Which part? The part about why I think the implementation should be
> > different? Or the part in which making a change causes future problems?
>
> The part where there is an ABI break that's insurmountable. But as I
> said, I need to read
> your patches with fresh eyes, not with past-midnight eyes.
Hyvää viikonloppua, Ville.
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel DPG Cloud Engineering
next prev parent reply other threads:[~2021-02-27 0:53 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-25 22:50 Thiago Macieira
2021-02-26 11:19 ` Jonathan Wakely
2021-02-26 17:37 ` Thiago Macieira
2021-02-26 18:29 ` Jonathan Wakely
2021-02-26 19:30 ` Ville Voutilainen
2021-02-26 21:17 ` Jonathan Wakely
2021-02-26 21:18 ` Ville Voutilainen
2021-02-26 21:39 ` Jonathan Wakely
2021-02-26 18:47 ` Ville Voutilainen
2021-02-26 23:53 ` Thiago Macieira
2021-02-26 23:58 ` Ville Voutilainen
2021-02-27 0:11 ` Thiago Macieira
2021-02-27 0:18 ` Ville Voutilainen
2021-02-27 0:36 ` Thiago Macieira
2021-02-27 0:44 ` Ville Voutilainen
2021-02-27 0:53 ` Thiago Macieira [this message]
2021-02-27 1:03 ` Ville Voutilainen
2021-03-03 14:30 ` Jonathan Wakely
2021-03-03 17:07 ` Thiago Macieira
2021-03-03 17:14 ` Jonathan Wakely
2021-02-27 0:22 ` Marc Glisse
2021-02-27 0:30 ` Ville Voutilainen
2021-02-27 0:43 ` Thiago Macieira
2021-03-03 14:24 ` Jonathan Wakely
2021-03-03 17:12 ` Thiago Macieira
2021-02-26 15:59 ` [PATCH 1/5] std::latch: reduce internal implementation from ptrdiff_t to int Thiago Macieira
2021-02-26 15:59 ` [PATCH 2/5] Atomic __platform_wait: accept any 32-bit type, not just int Thiago Macieira
2021-03-03 14:34 ` Jonathan Wakely
2021-03-03 16:21 ` Jonathan Wakely
2021-03-03 17:27 ` Thiago Macieira
2021-03-03 17:34 ` Ville Voutilainen
2021-03-03 17:41 ` Jonathan Wakely
2021-02-26 15:59 ` [PATCH 3/5] std::__atomic_wait: don't use __detail::__waiter with futex Thiago Macieira
2021-02-26 15:59 ` [PATCH 4/5] barrier: use int instead of unsigned char for the phase state Thiago Macieira
2021-02-28 15:05 ` Hans-Peter Nilsson
2021-03-01 16:28 ` Thomas Rodgers
2021-03-01 17:24 ` Thiago Macieira
2021-03-01 17:38 ` Thomas Rodgers
2021-03-01 17:40 ` Thomas Rodgers
2021-03-01 18:06 ` Thiago Macieira
2021-03-01 19:08 ` Thomas Rodgers
2021-03-01 18:12 ` Ville Voutilainen
2021-03-01 19:44 ` Thiago Macieira
2021-03-01 20:35 ` Ville Voutilainen
2021-03-01 21:54 ` Thiago Macieira
2021-03-01 22:04 ` Ville Voutilainen
2021-03-01 22:21 ` Thiago Macieira
2021-03-01 22:31 ` Ville Voutilainen
2021-03-01 22:40 ` Thiago Macieira
2021-02-26 15:59 ` [PATCH 5/5] barrier: optimise by not having the hasher in a loop Thiago Macieira
2021-03-03 14:36 ` Jonathan Wakely
2021-02-26 18:14 ` [PATCH 1/5] std::latch: reduce internal implementation from ptrdiff_t to int Andreas Schwab
2021-02-26 19:08 ` Thiago Macieira
2021-02-26 19:31 ` Andreas Schwab
2021-02-27 0:13 ` Thiago Macieira
2021-02-28 21:31 ` Hans-Peter Nilsson
2021-03-01 8:56 ` Richard Biener
2021-03-03 14:56 ` Jonathan Wakely
2021-03-03 15:02 ` Andreas Schwab
2021-03-03 15:10 ` Jonathan Wakely
2021-03-03 15:37 ` Hans-Peter Nilsson
2021-03-01 16:32 ` Thomas Rodgers
2021-03-03 14:34 ` Jonathan Wakely
2021-03-03 17:14 ` Thiago Macieira
2021-03-03 17:18 ` Jonathan Wakely
2021-02-27 1:13 ` C++2a synchronisation inefficient in GCC 11 Thomas Rodgers
2021-02-27 1:29 ` Thomas Rodgers
2021-02-27 3:01 ` Thomas Rodgers
2021-03-01 17:46 ` Thomas Rodgers
2021-03-01 18:00 ` Thiago Macieira
2021-03-01 18:34 ` Thomas Rodgers
2021-03-01 19:11 ` Thiago Macieira
2021-02-27 2:02 ` Ville Voutilainen
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