Lec10 Threads

7/25/2019 Lec10 Threads

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ASIC Verification

Threads



Topics

• Threads

• fork..join, fork…join_any, fork…join_none

• Creating dynamic threads

• Using automatic variables

• Process control



Working with Threads

•Grouping Statements Classic Verilog has two ways of grouping statements

Using begin…end : Statements run sequentially

Using fork…join : Statements run in parallel

• fork…join

fork…join construct enables the creation of concurrent processes from

each of its parallel statements




•

join, join_any, join_none SystemVerilog uses three ways for specifying when the parent process

resumes execution

fork…join

fork…join_any

fork…join_none

fork

join

fork fork

join_any join_none




•

fork…join The parent process blocks until all the processes spawned by this fork

complete

When defining a fork…join block, encapsulating the entire fork within a

begin…end block causes the entire block to execute as a single process,

with each statement executing sequentially

• fork…join example

fork begin

$display( "First Block\n" );

# 20ns;

end begin

$display( "Second Block\n" );

@eventA;

end join

Process 1

Process 2




•

fork…join example (cont.) In the example two processes are forked

The first process waits for 20 ns

The second process waits for the named event eventA to be triggered

Because the join keyword is specified, the parent process shall block until

the two processes complete; that is, until 20ns have elapsed and eventA

has been triggered fork

join

begin $display("First Block \n”);

# 20ns;

end

begin $display(”Second Block \n”);

@eventA;

end




•

fork…join_any The parent process blocks until any one of the processes spawned by this

fork complete

• fork…join_any example

fork begin


# 20ns;

end begin


@eventA;

end join_any

Process 1

Process 2




•

fork…join_any example (cont.) In the example two processes are forked



Because the join_any keyword is specified, the parent process shall block

until any one of the two processes complete; that is, until 20ns have

elapsed or eventA has been triggered

fork

join_any

begin

$display("First Block \n”);# 20ns;

end

begin

$display(”Second Block \n”);@eventA;

end




•

fork…join_none The parent process continues to execute concurrently with all the processes

spawned by the fork. The spawned processes do not start executing until

the parent thread executes a blocking statement

• fork…join_none example

fork begin


# 20ns;

end begin


@eventA;

end join_none

Process 1

Process 2




•

fork…join_none example (cont.) In the example two processes are forked



Because the join_none keyword is specified, the parent process shall

continue to execute with the other two processes.

fork

join_none

begin $display("First Block \n”);

# 20ns;

end

begin $display(”Second Block \n”);

@eventA;

end




•

fork…join and begin…end example

initial begin $display(“@%0d: starts fork…join example”,$time);

#10 $display(“@%0d: sequential after #10”,$time);

fork

$display(“@%0d: parallel start”,$time); #50 $display(“@%0d: parallel after #50”,$time);

#10 $display(“@%0d: parallel after #10”,$time);

begin #30 $display(“@%0d: sequential after #30”,$time);


end join $display(“@%0d: after join”,$time);

#80 $display(“@%0d: final after #80”,$time);

end

Parent

Parent

Child Threads




• fork…join and begin…end example

Parent

Child Threads

@0: starts fork…join example

@10: sequential after #10@10: parallel start@20: parallel after #10@40: sequential after #30@50: sequential after #10@60: parallel after #50@60: after join@140: final after #80

$display(“@%0d: starts fork…join example”,$time);#10 $display(“@%0d: sequential after #10”,$time);

$display(“@%0d: parallel start”,$time);





$display(“@%0d: after join”,$time);

#80 $display(“@%0d: final after #80”,$time); Parent




•

fork…join_any and begin…end example

initial begin $display(“@%0d: starts fork…join_any example”,$time);


fork





end join_any $display(“@%0d: after join”,$time);


end

Parent

Parent

Child Threads




• fork…join_any and begin…end example

Parent

Child Threads

@0: starts fork…join_any example

@10: sequential after #10@10: parallel start@10: after join@20: parallel after #10@40: sequential after #30@50: sequential after #10@60: parallel after #50@90: final after #80

$display(“@%0d: starts fork…join_any example”,$time);#10 $display(“@%0d: sequential after #10”,$time);











•

fork…join_none and begin…end example

initial begin $display(“@%0d: starts fork…join_none example”,$time);


fork





end join_none $display(“@%0d: after join”,$time);


end

Parent

Parent

Child Threads




• fork…join_none and begin…end example

Parent

Child Threads

@0: starts fork…join_none example

@10: sequential after #10@10: after join@10: parallel start@20: parallel after #10@40: sequential after #30@50: sequential after #10@60: parallel after #50@90: final after #80

$display(“@%0d: starts fork…join_none example”,$time);#10 $display(“@%0d: sequential after #10”,$time);










Automatic Variables in Threads

• Avoiding Bugs

program no_auto;

intial begin

for(int j=0; j<3; j++)

fork

$write(j);

join_none

#0 $display (“\n”); end

endprogram

j statement

0 for (j=0; …

0 Spawn $write j [thread 0]

1 j++ j=1


2 j++ j=2


3 j++ j=3

3 join_none

3 #0

3 $write(j) [thread 0: j=3]



3 $display()



Automatic Variables in Threads

• Avoiding Bugs

Program auto;intial begin

for(int j=0; j<3; j++)

fork

automatic int k=j;$write(k);

join_none

#0 $display (“\n”); end

endprogram

j k0 k1 k2 statement

0 for (j=0; …

0 0 Create k0, Spawn $write k

[thread 0]

1 0 j++

1 0 1 Create k1, Spawn $write k

[thread1]

2 0 1 j++

2 0 1 2 Create k2, Spawn $write k

[thread 2]

3 0 1 2 j<3

3 0 1 2 join_none

3 0 1 2 #0

3 0 1 2 $write(k0) [thread 0]



3 0 1 2 $display()



Dynamic Thread Creation


program test(busif.TB bus);task check_trans(Transaction tr);

fork

begin

wait (bus.cb.addr != tr.addr);

$display("@%0d: Addr match %d", $time, tr.addr);

end

join_none

endtask

Transaction tr;

initial begin

repeat (10)

begin

tr = new();assert(tr.randomize);

transmit(tr);

check_trans(tr);

end

#100

end

endprogram

Create a random transaction

Send it into the DUT

Wait for reply from DUT



Dynamic Thread Creation


program automatic test(busif.TB bus);task check_trans(Transaction tr);

fork

begin


$display("@%0d: Addr match %d", $time, tr.addr);

end

join_none

endtask

Transaction tr;

initial begin

repeat (10)

begin

tr = new();assert(tr.randomize);

transmit(tr);

check_trans(tr);

end

#100

end

endprogram

Create a random transaction

Send it into the DUT

Wait for reply from DUT



Threads: Process Control

• wait…fork

SystemVerilog provides a construct wait fork that waits for thecompletion of other processes

The wait fork is used to ensure that all child processes (processes

created by the calling process) have completed their execution

Specifying a wait fork causes the calling process to block until all its

sub-processes have completed

task run_threads;fork check_trans(tr1);

check_trans(tr2);

check_trans(tr3);

join_none wait fork;

endtask

wait fork statement shall ensure

that the task run_threads waits for all

spawned processes to complete

before returning to its caller

Block until

check_trans(tr1),

check_trans(tr2),

check_trans(tr3) complete




• disable…fork

SystemVerilog provides a construct disable fork that stops theexecution of processes

The disable fork statement terminates all descendants of the calling

process, as well as the descendants of the process descendants, that is, ifany child process has descendants of their own, the disable fork

statement terminates them as well

task get_first( output int addr );fork wait_device( 1, addr );

wait_device( 7, addr );

wait_device( 13, addr );

join_any disable fork;

endtask

In the example get_first spawns three

versions of a task that wait for a particulardevice (1,7 or 13). The task wait_device

waits for a particular device to become

ready and then returns the device’s

address. When the first device becomesavailable, the get_first task shall

resume execution and proceed to kill theoutstanding wait_device request

Disable ends all processes whereas disable fork only ends the process spawned by the calling thread




program test(busif.TB bus);

task check_trans(Transaction tr);fork

begin


end

join_none

endtask

Transaction tr;

initial begin

check_trans(tr0);

fork

begin

check_trans(tr1);

forkcheck_trans(tr2);

join

#(TIME_OUT) disable fork;

end

join

end

endprogram

initial begin

check_trans(tr0);

fork

…

join

end

Thread 0

Thread 1

check_trans(tr1);

fork

…

joinend

#TIME_OUT/2

disable fork

Thread 2

Thread 3check_trans(tr2);

Thread 4

• disable…fork




initial begin

check_trans(tr0);

fork

begin : threads_innercheck_trans(tr1);

check_trans(tr2);

end

#(TIME_OUT/2) disable threads_inner;

join

end

endprogram

• disable…fork: with explicit labels



Thank You

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