<s>
Scheduler	B-Operating_System
activations	I-Operating_System
are	O
a	O
threading	O
mechanism	O
that	O
,	O
when	O
implemented	O
in	O
an	O
operating	B-General_Concept
system	I-General_Concept
's	O
process	O
scheduler	O
,	O
provide	O
kernel-level	O
thread	B-Operating_System
functionality	O
with	O
user-level	O
thread	B-Operating_System
flexibility	O
and	O
performance	O
.	O
</s>
<s>
This	O
mechanism	O
uses	O
a	O
so-called	O
"	O
N:M	O
"	O
strategy	O
that	O
maps	O
some	O
N	O
number	O
of	O
application	O
threads	B-Operating_System
onto	O
some	O
M	O
number	O
of	O
kernel	B-Operating_System
entities	O
,	O
or	O
"	O
virtual	O
processors.	O
"	O
</s>
<s>
This	O
is	O
a	O
compromise	O
between	O
kernel-level	O
(	O
"	O
1:1	O
"	O
)	O
and	O
user-level	O
(	O
"	O
N:1	O
"	O
)	O
threading	O
.	O
</s>
<s>
In	O
general	O
,	O
"	O
N:M	O
"	O
threading	O
systems	O
are	O
more	O
complex	O
to	O
implement	O
than	O
either	O
kernel	B-Operating_System
or	O
user	O
threads	B-Operating_System
,	O
because	O
both	O
changes	O
to	O
kernel	B-Operating_System
and	O
user-space	O
code	O
are	O
required	O
.	O
</s>
<s>
Scheduler	B-Operating_System
activations	I-Operating_System
were	O
proposed	O
by	O
Anderson	O
,	O
Bershad	O
,	O
Lazowska	O
,	O
and	O
Levy	O
in	O
Scheduler	B-Operating_System
Activations	I-Operating_System
:	O
Effective	O
Kernel	B-Operating_System
Support	O
for	O
the	O
User-Level	O
Management	O
of	O
Parallelism	O
in	O
1991	O
.	O
</s>
<s>
Support	O
was	O
implemented	O
in	O
the	O
NetBSD	B-Device
kernel	I-Device
by	O
Nathan	O
Williams	O
but	O
has	O
since	O
been	O
abandoned	O
in	O
favor	O
of	O
1:1	O
threading	O
.	O
</s>
<s>
FreeBSD	B-Operating_System
had	O
a	O
similar	O
threading	O
implementation	O
called	O
Kernel	B-Operating_System
Scheduled	O
Entities	O
which	O
is	O
also	O
being	O
retired	O
in	O
favor	O
of	O
1:1	O
threading	O
.	O
</s>
<s>
Scheduler	B-Operating_System
activations	I-Operating_System
were	O
also	O
implemented	O
as	O
a	O
patch	O
for	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
by	O
Vincent	O
Danjean	O
:	O
Linux	B-Operating_System
Activations	O
,	O
the	O
user-level	O
part	O
being	O
done	O
in	O
the	O
Marcel	O
thread	B-Operating_System
library	O
.	O
</s>
