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GDC - program runs in one thread, DMD - in 4 threads, why?
Sep 10
eugene
Sep 10
drug
Sep 10
eugene
Sep 10
Basile B.
Sep 10
bauss
Sep 10
eugene
Sep 10
eugene
Sep 10
eugene
Sep 10
eugene
Sep 10
eugene
Sep 10
bauss
Sep 10
eugene
September 10

Here is test program (which is using DList aggressively)

import std.stdio : writeln;
import core.sys.posix.unistd : sleep;
//import std.container.dlist; // dmd (v2.097.2)
import std.container: DList; // gdc (4.9.2)

void main() {

    auto list = DList!(int)();
    int k;

    while (true) {
        // 1.sleep;
        k.writeln;
        list.insertBack(k);
        list.insertBack(k);
        list.removeFront();
        list.removeFront();
        k++;
    }
}

When compiled with gdc, it runs in one thread:

@dexp ~ $ ps xH | grep [t]est
3823 pts/12 R+ 0:07 ./test

But when with dmd, program creates 3 additional threads:

@dexp ~ $ ps xH | grep [t]est
3839 pts/12 Rl+ 0:00 ./test
3839 pts/12 Sl+ 0:00 ./test
3839 pts/12 Sl+ 0:00 ./test
3839 pts/12 Sl+ 0:00 ./test

What are these extra threads for?
To work with list (which is strange)???
GC?
Something else?

Observations:

  • without DList there is no extra-threads
  • with sleep() there is also no extra-threads

So I think it is very aggressive usage of DList that causes this.

Can this (really unwanted) behavior be disabled in DMD?
I do not want to have multiple threads,
a program (real program, not the test above) has to be single-threaded.

system used:
@dexp ~ $ cat /etc/debian_version
8.11

September 10
10.09.2021 12:27, eugene пишет:

> //import std.container.dlist; // dmd (v2.097.2)
> import std.container: DList; // gdc (4.9.2)
> 


It is off-topic a bit but I think none can compare gdc 4.9.2 to dmd 2.097.2 because gdc has older version than dmd. I would compare gdc to appropriate dmd version, it's about 2.078. But note that gdc backported some code from more recent dmd-fe versions so it has 2.078-dirty version in fact.
September 10

On Friday, 10 September 2021 at 09:27:49 UTC, eugene wrote:

>

Here is test program (which is using DList aggressively)
[...]
Can this (really unwanted) behavior be disabled in DMD?
I do not want to have multiple threads,
a program (real program, not the test above) has to be single-threaded.

system used:
@dexp ~ $ cat /etc/debian_version
8.11

most recent dmd has a GC that collect in threads. it can be tuned but I dont find the documentation for this right now.

September 10

On Friday, 10 September 2021 at 10:39:48 UTC, Basile B. wrote:

>

On Friday, 10 September 2021 at 09:27:49 UTC, eugene wrote:

>

Here is test program (which is using DList aggressively)
[...]
Can this (really unwanted) behavior be disabled in DMD?
I do not want to have multiple threads,
a program (real program, not the test above) has to be single-threaded.

system used:
@dexp ~ $ cat /etc/debian_version
8.11

most recent dmd has a GC that collect in threads. it can be tuned but I dont find the documentation for this right now.

Here's the specific change:

https://dlang.org/changelog/2.087.0.html#gc_parallel

GC now marks the heap with multiple threads

The garbage collector now uses available CPU cores to mark the heap faster. This reduces pause times for a collection considerably.

By default, the GC uses all available logical cores of your CPU. This might affect your application if it has threads that are not suspended during the mark phase of the collection. You can configure the number of additional threads used for marking by DRT option parallel to the GC configuration, e.g. by passing --DRT-gcopt=parallel:2 on the command line. A value of 0 disables parallel marking completely.

As usual, you can also embed the configuration into the application by redefining rt_options, e.g.

extern(C) __gshared string[] rt_options = [ "gcopt=parallel:0" ];

So for OP what you want to use is:

--DRT-gcopt=parallel:0
September 10
On Friday, 10 September 2021 at 10:20:52 UTC, drug wrote:
> It is off-topic a bit

I am newbie - have been learning D for about 2 months or so.
I understand that my question is not about the language itself,
just picked forum for new users.

> but I think none can compare gdc 4.9.2 to

same picture with gdc 8.4.0 - one thread, no pthread_create() behind the scenes.


September 10
On Friday, 10 September 2021 at 11:20:10 UTC, eugene wrote:

>
> same picture with gdc 8.4.0 - one thread, no pthread_create() behind the scenes.

GDC is stuck on a much older version of D. Iain has backported some bugfixes and optimizations, but featurewise it's mostly D 2.076. This is because that's the last version of the D frontend implemented in C++, and he needed that to get gdc into gcc. As bauss pointed out, the library feature you're talking about was implemented in 2.087. Iain is working on porting gdc over to the D version of the D frontend, at which point it will be much more up-to-date.
September 10

On Friday, 10 September 2021 at 11:09:10 UTC, bauss wrote:

>

Here's the specific change:
https://dlang.org/changelog/2.087.0.html#gc_parallel

thanx a lot!

September 10
On Friday, 10 September 2021 at 09:27:49 UTC, eugene wrote:
> What are these extra threads for?
> GC?
>
> So I think it is **very aggressive usage** of DList that causes this.

Yeah, in newer versions, when the garbage collector does its first collect, it spawns some helper threads to speed up its mark process. When the GC is not actively marking, those extra threads are 100% idle, so they don't affect the rest of your program.

> Can this (really unwanted) behavior be disabled in DMD?
> I do not want to have multiple threads,

here's the page on it
https://dlang.org/spec/garbage.html#gc_config

You either pass as an argument *to your application*

--DRT-gcopt=parallel:0

or to make it permanent, in your source file with the main() function, add this:

extern(C) __gshared string[] rt_options = [ "gcopt=parallel:0" ];


and it will stop doing that.
September 10
On Friday, 10 September 2021 at 11:09:10 UTC, bauss wrote:
> --DRT-gcopt=parallel:2 on the command line. A value of 0 disables parallel marking completely.

but it does not:

make -f Makefile-dmd
dmd --DRT-gcopt=parallel:0 engine/*.d common-sm/*.d server-sm/*.d pool.d echo_server.d -ofecho-server
dmd --DRT-gcopt=parallel:0 engine/*.d common-sm/*.d client-sm/*.d pool.d echo_client.d -ofecho-client

ps xH | grep [e]cho
 5460 pts/14   Sl+    0:00 ./echo-server
 5460 pts/14   Sl+    0:00 ./echo-server
 5460 pts/14   Sl+    0:00 ./echo-server
 5460 pts/14   Sl+    0:00 ./echo-server
 5466 pts/15   Sl+    0:00 ./echo-client
 5466 pts/15   Sl+    0:00 ./echo-client
 5466 pts/15   Sl+    0:00 ./echo-client
 5466 pts/15   Sl+    0:00 ./echo-client



September 10
On Friday, 10 September 2021 at 11:32:02 UTC, Adam D Ruppe wrote:
> You either pass as an argument *to your application*
> --DRT-gcopt=parallel:0

oops... :)

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