By: anon (anon.delete@this.anon.com), August 21, 2018 3:27 pm
Room: Moderated Discussions
Linus Torvalds (torvalds.delete@this.linux-foundation.org) on August 21, 2018 3:31 pm wrote:
> Linus Torvalds (torvalds.delete@this.linux-foundation.org) on August 21, 2018 2:34 pm wrote:
> >
> > I Again, rseq is designed for per-cpu lockless algorithms. The main use case is
> > literally a percpu memory allocator that doesn't need to take locks. There are others, but
> > memory allocators can be so critical that you might as well see that as the primary one.
>
> Actually, another thing worth mentioning is just "statistics".
>
> It sounds ridiculously easy to do things like "count number of packets processed" or
> "count hash misses vs hits", but it turns out that in heavily threaded environments, doing
> things like operation statistics is sometimes some of the nastiest stuff there is.
>
> It's trivial to do per-thread: just increment the counter in the per-thread area. Boom. Done.
> There are no races, because even on the worst kind of load-store machine you can just do
>
>
> and you're done. No locks, no cache conflicts with other threads, no nothing.
>
> The problem with the per-thread model is that it can be very expensive indeed to add all those per-thread
> statistics up. Thousands of threads is not even unusual. So maybe you can deal with some slop, and just have
> the threads periodically empoty their per-thread statistics into some cache-coherent pool or something.
>
> It's solvable, don't get me wrong. But it has problems.
>
> So you can solve it other ways. One way is to have the remote atomics, and just do the statistics update
> "somewhere else" and asynchronously. That's a lovely model in many ways, but it's not portable.
"Remote atomics"? "Lovely model"? Who is this crazy person impersonating Linus?
I assume you mean each core sending a non-cache-coherent message off core somewhere (a memory controller?) which atomically increments a single location. That's crazy. First of all you have the same scaling problem, now you just offloaded it to the non-coherent messaging system and the entity performing the remote atomic. Secondly, high performance use cases that would care (like networking) literally count cache misses (or any type of operation that goes out of the core). Adding one more per packet might cut your performance by double digit percentages.
And no it often doesn't really matter if they are asynchronous because at the rates involved, and when interconnects and memory traffic is hot, and with any amount of contention, these queues are going to get backlogged.
Just don't do it. Asking CPU designers to do something like that is like giving them a rope you've tied around your own neck and telling them that if they tie the other end to their neck then you both jump off a cliff together, you'll finally reach the end of the magical leprechaun's rainbow. And they'd happily do it too.
No, just tell them to make the basics work fast and reduce the corner cases and pathologial cases. If a CPU designer starts telling you about remote atomics, tell them instead to spend all that research and development effort on making local atomics 5% faster. I guarantee that will be more useful.
> Linus Torvalds (torvalds.delete@this.linux-foundation.org) on August 21, 2018 2:34 pm wrote:
> >
> > I Again, rseq is designed for per-cpu lockless algorithms. The main use case is
> > literally a percpu memory allocator that doesn't need to take locks. There are others, but
> > memory allocators can be so critical that you might as well see that as the primary one.
>
> Actually, another thing worth mentioning is just "statistics".
>
> It sounds ridiculously easy to do things like "count number of packets processed" or
> "count hash misses vs hits", but it turns out that in heavily threaded environments, doing
> things like operation statistics is sometimes some of the nastiest stuff there is.
>
> It's trivial to do per-thread: just increment the counter in the per-thread area. Boom. Done.
> There are no races, because even on the worst kind of load-store machine you can just do
>
>
get thread pointer
> load counter from memory off thread pointer
> increment counter
> store new counter off thread pointer
> and you're done. No locks, no cache conflicts with other threads, no nothing.
>
> The problem with the per-thread model is that it can be very expensive indeed to add all those per-thread
> statistics up. Thousands of threads is not even unusual. So maybe you can deal with some slop, and just have
> the threads periodically empoty their per-thread statistics into some cache-coherent pool or something.
>
> It's solvable, don't get me wrong. But it has problems.
>
> So you can solve it other ways. One way is to have the remote atomics, and just do the statistics update
> "somewhere else" and asynchronously. That's a lovely model in many ways, but it's not portable.
"Remote atomics"? "Lovely model"? Who is this crazy person impersonating Linus?
I assume you mean each core sending a non-cache-coherent message off core somewhere (a memory controller?) which atomically increments a single location. That's crazy. First of all you have the same scaling problem, now you just offloaded it to the non-coherent messaging system and the entity performing the remote atomic. Secondly, high performance use cases that would care (like networking) literally count cache misses (or any type of operation that goes out of the core). Adding one more per packet might cut your performance by double digit percentages.
And no it often doesn't really matter if they are asynchronous because at the rates involved, and when interconnects and memory traffic is hot, and with any amount of contention, these queues are going to get backlogged.
Just don't do it. Asking CPU designers to do something like that is like giving them a rope you've tied around your own neck and telling them that if they tie the other end to their neck then you both jump off a cliff together, you'll finally reach the end of the magical leprechaun's rainbow. And they'd happily do it too.
No, just tell them to make the basics work fast and reduce the corner cases and pathologial cases. If a CPU designer starts telling you about remote atomics, tell them instead to spend all that research and development effort on making local atomics 5% faster. I guarantee that will be more useful.
Topic | Posted By | Date |
---|---|---|
ARM turns to a god and a hero | AM | 2018/08/16 08:32 AM |
ARM turns to a god and a hero | Maynard Handley | 2018/08/16 08:41 AM |
ARM turns to a god and a hero | Doug S | 2018/08/16 10:11 AM |
ARM turns to a god and a hero | Geoff Langdale | 2018/08/16 10:59 PM |
ARM turns to a god and a hero | dmcq | 2018/08/17 04:12 AM |
ARM is somewhat misleading | Adrian | 2018/08/16 10:56 PM |
It's marketing material | Gabriele Svelto | 2018/08/17 12:00 AM |
It's marketing material | Michael S | 2018/08/17 02:13 AM |
It's marketing material | dmcq | 2018/08/17 04:23 AM |
It's marketing material | Andrei Frumusanu | 2018/08/17 06:25 AM |
It's marketing material | Linus Torvalds | 2018/08/17 10:20 AM |
It's marketing material | Groo | 2018/08/17 12:44 PM |
It's marketing material | Doug S | 2018/08/17 01:14 PM |
promises and deliveries | AM | 2018/08/17 01:32 PM |
promises and deliveries | Passing Through | 2018/08/17 02:02 PM |
Just by way of clarification | Passing Through | 2018/08/17 02:15 PM |
Just by way of clarification | AM | 2018/08/18 11:49 AM |
Just by way of clarification | Passing Through | 2018/08/18 12:34 PM |
This ain't the nineties any longer | Passing Through | 2018/08/18 12:54 PM |
This ain't the nineties any longer | Maynard Handley | 2018/08/18 01:50 PM |
This ain't the nineties any longer | Passing Through | 2018/08/18 02:57 PM |
This ain't the nineties any longer | Passing Through | 2018/09/06 01:42 PM |
This ain't the nineties any longer | Maynard Handley | 2018/09/07 03:10 PM |
This ain't the nineties any longer | Passing Through | 2018/09/07 03:48 PM |
This ain't the nineties any longer | Maynard Handley | 2018/09/07 04:22 PM |
Just by way of clarification | Wilco | 2018/08/18 12:26 PM |
Just by way of clarification | Passing Through | 2018/08/18 12:39 PM |
Just by way of clarification | none | 2018/08/18 09:52 PM |
Just by way of clarification | dmcq | 2018/08/19 07:32 AM |
Just by way of clarification | none | 2018/08/19 07:54 AM |
Just by way of clarification | dmcq | 2018/08/19 10:24 AM |
Just by way of clarification | none | 2018/08/19 10:52 AM |
Just by way of clarification | Gabriele Svelto | 2018/08/19 05:41 AM |
Just by way of clarification | Passing Through | 2018/08/19 08:25 AM |
Whiteboards at Gatwick airport anyone? | Passing Through | 2018/08/20 03:24 AM |
It's marketing material | Michael S | 2018/08/18 10:12 AM |
It's marketing material | Brett | 2018/08/18 04:22 PM |
It's marketing material | Brett | 2018/08/18 04:33 PM |
It's marketing material | Adrian | 2018/08/19 12:21 AM |
A76 | AM | 2018/08/17 01:45 PM |
A76 | Michael S | 2018/08/18 10:20 AM |
A76 | AM | 2018/08/18 11:39 AM |
A76 | Michael S | 2018/08/18 11:49 AM |
A76 | AM | 2018/08/18 12:06 PM |
A76 | Doug S | 2018/08/18 12:43 PM |
A76 | Maynard Handley | 2018/08/18 01:42 PM |
A76 | Maynard Handley | 2018/08/18 03:22 PM |
Why write zeros when one can use metadata? | Paul A. Clayton | 2018/08/18 05:19 PM |
Why write zeros when one can use metadata? | Maynard Handley | 2018/08/19 10:12 AM |
Dictionary compress might apply to memcopy | Paul A. Clayton | 2018/08/19 12:45 PM |
Instructions for zeroing | Konrad Schwarz | 2018/08/30 05:37 AM |
Instructions for zeroing | Maynard Handley | 2018/08/30 07:41 AM |
Instructions for zeroing | Adrian | 2018/08/30 10:37 AM |
dcbz -> dcbzl (was: Instructions for zeroing) | hobold | 2018/08/31 12:50 AM |
dcbz -> dcbzl (was: Instructions for zeroing) | dmcq | 2018/09/01 04:28 AM |
A76 | Travis | 2018/08/19 10:36 AM |
A76 | Maynard Handley | 2018/08/19 11:22 AM |
A76 | Travis | 2018/08/19 01:07 PM |
A76 | Maynard Handley | 2018/08/19 05:24 PM |
Remote atomics | matthew | 2018/08/19 11:51 AM |
Remote atomics | Michael S | 2018/08/19 12:58 PM |
Remote atomics | matthew | 2018/08/19 01:32 PM |
Remote atomics | Michael S | 2018/08/19 01:36 PM |
Remote atomics | matthew | 2018/08/19 01:48 PM |
Remote atomics | Michael S | 2018/08/19 02:16 PM |
Remote atomics | Ricardo B | 2018/08/20 09:05 AM |
Remote atomics | dmcq | 2018/08/19 01:33 PM |
Remote atomics | Travis | 2018/08/19 01:32 PM |
Remote atomics | Michael S | 2018/08/19 01:46 PM |
Remote atomics | Travis | 2018/08/19 04:35 PM |
Remote atomics | Michael S | 2018/08/20 02:29 AM |
Remote atomics | matthew | 2018/08/19 06:58 PM |
Remote atomics | anon | 2018/08/19 11:59 PM |
Remote atomics | Travis | 2018/08/20 09:26 AM |
Remote atomics | Travis | 2018/08/20 08:57 AM |
Remote atomics | Linus Torvalds | 2018/08/20 03:29 PM |
Fitting time slices to execution phases | Paul A. Clayton | 2018/08/21 08:09 AM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 01:34 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 02:31 PM |
Fitting time slices to execution phases | Gabriele Svelto | 2018/08/21 02:54 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 03:26 PM |
Fitting time slices to execution phases | Travis | 2018/08/21 03:21 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 03:39 PM |
Fitting time slices to execution phases | Travis | 2018/08/21 03:59 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 04:13 PM |
Fitting time slices to execution phases | anon | 2018/08/21 03:27 PM |
Fitting time slices to execution phases | Linus Torvalds | 2018/08/21 05:02 PM |
Fitting time slices to execution phases | Etienne | 2018/08/22 01:28 AM |
Fitting time slices to execution phases | Gabriele Svelto | 2018/08/22 02:07 PM |
Fitting time slices to execution phases | Travis | 2018/08/22 03:00 PM |
Fitting time slices to execution phases | anon | 2018/08/22 05:52 PM |
Fitting time slices to execution phases | Travis | 2018/08/21 03:37 PM |
Is preventing misuse that complex? | Paul A. Clayton | 2018/08/23 04:42 AM |
Is preventing misuse that complex? | Linus Torvalds | 2018/08/23 11:46 AM |
Is preventing misuse that complex? | Travis | 2018/08/23 12:29 PM |
Is preventing misuse that complex? | Travis | 2018/08/23 12:33 PM |
Is preventing misuse that complex? | Jeff S. | 2018/08/24 06:57 AM |
Is preventing misuse that complex? | Travis | 2018/08/24 07:47 AM |
Is preventing misuse that complex? | Linus Torvalds | 2018/08/23 01:30 PM |
Is preventing misuse that complex? | Travis | 2018/08/23 02:11 PM |
Is preventing misuse that complex? | Linus Torvalds | 2018/08/24 12:00 PM |
Is preventing misuse that complex? | Gabriele Svelto | 2018/08/24 12:25 PM |
Is preventing misuse that complex? | Linus Torvalds | 2018/08/24 12:33 PM |
Fitting time slices to execution phases | Travis | 2018/08/21 02:54 PM |
rseq: holy grail rwlock? | Travis | 2018/08/21 02:18 PM |
rseq: holy grail rwlock? | Linus Torvalds | 2018/08/21 02:59 PM |
rseq: holy grail rwlock? | Travis | 2018/08/21 03:27 PM |
rseq: holy grail rwlock? | Linus Torvalds | 2018/08/21 04:10 PM |
rseq: holy grail rwlock? | Travis | 2018/08/21 05:21 PM |
ARM design houses | Michael S | 2018/08/21 04:07 AM |
ARM design houses | Wilco | 2018/08/22 11:38 AM |
ARM design houses | Michael S | 2018/08/22 01:21 PM |
ARM design houses | Wilco | 2018/08/22 02:23 PM |
ARM design houses | Michael S | 2018/08/29 12:58 AM |
Qualcomm's core naming scheme really, really sucks | Heikki Kultala | 2018/08/29 01:19 AM |
A76 | Maynard Handley | 2018/08/18 01:07 PM |
A76 | Michael S | 2018/08/18 01:32 PM |
A76 | Maynard Handley | 2018/08/18 01:52 PM |
A76 | Michael S | 2018/08/18 02:04 PM |
ARM is somewhat misleading | juanrga | 2018/08/17 12:20 AM |
Surprised?? | Alberto | 2018/08/17 12:52 AM |
Surprised?? | Alberto | 2018/08/17 01:10 AM |
Surprised?? | none | 2018/08/17 01:46 AM |
Garbage talk | Andrei Frumusanu | 2018/08/17 06:30 AM |
Garbage talk | Michael S | 2018/08/17 06:43 AM |
Garbage talk | Andrei Frumusanu | 2018/08/17 08:51 AM |
Garbage talk | Michael S | 2018/08/18 10:29 AM |
Garbage talk | Adrian | 2018/08/17 07:28 AM |
Garbage talk | Alberto | 2018/08/17 08:20 AM |
Garbage talk | Andrei Frumusanu | 2018/08/17 08:48 AM |
Garbage talk | Adrian | 2018/08/17 09:17 AM |
Garbage talk | Andrei Frumusanu | 2018/08/17 09:36 AM |
Garbage talk | Adrian | 2018/08/17 01:53 PM |
Garbage talk | Andrei Frumusanu | 2018/08/17 11:17 PM |
More like a religion he?? ARM has an easy life :) | Alberto | 2018/08/17 08:13 AM |
More like a religion he?? ARM has an easy life :) | Andrei Frumusanu | 2018/08/17 08:34 AM |
More like a religion he?? ARM has an easy life :) | Alberto | 2018/08/17 09:03 AM |
More like a religion he?? ARM has an easy life :) | Andrei Frumusanu | 2018/08/17 09:43 AM |
More like a religion he?? ARM has an easy life :) | Doug S | 2018/08/17 01:17 PM |
15W phone SoCs | AM | 2018/08/17 02:04 PM |
More like a religion he?? ARM has an easy life :) | Maynard Handley | 2018/08/17 11:29 AM |
my future stuff will be better than your old stuff, hey I'm a god at last (NT) | Eric Bron | 2018/08/18 02:34 AM |
my future stuff will be better than your old stuff, hey I'm a god at last | none | 2018/08/18 07:34 AM |