mt-streams(9F)						   Kernel Functions for Drivers 					    mt-streams(9F)

NAME

       mt-streams - STREAMS multithreading

SYNOPSIS

       #include <sys/conf.h>

INTERFACE LEVEL

       Solaris DDI specific (Solaris DDI).

DESCRIPTION

       STREAMS	drivers  configures  the degree of concurrency using the cb_flag field in the cb_ops structure (see cb_ops(9S)). The corresponding
       field for STREAMS modules is the f_flag in the fmodsw structure.

       For the purpose of restricting and controlling the concurrency in drivers/modules, we define the concepts of inner and outer perimeters.  A
       driver/module  can be configured either to have no perimeters, to have only an inner or an outer perimeter, or to have both an inner and an
       outer perimeter. Each perimeter acts as a readers-writers lock, that is, there can be multiple concurrent readers or a single writer. Thus,
       each  perimeter can be entered in two modes: shared (reader) or exclusive (writer). The mode depends on the perimeter configuration and can
       be different for the different STREAMS entry points ( open(9E), close(9E), put(9E), or srv(9E)).

       The concurrency for the different entry points is (unless specified otherwise) to enter with exclusive access at the  inner  perimeter  (if
       present) and shared access at the outer perimeter (if present).

       The  perimeter configuration consists of flags that define the presence and scope of the inner perimeter, the presence of the outer perime-
       ter (which can only have one scope), and flags that modify the default concurrency for the different entry points.

       All MT safe modules/drivers specify the D_MP flag.

   Inner Perimeter Flags
       The inner perimeter presence and scope are controlled by the mutually exclusive flags:

       D_MTPERQ 	    The module/driver has an inner perimeter around each queue.

       D_MTQPAIR	    The module/driver has an inner perimeter around each read/write pair of queues.

       D_MTPERMOD	    The module/driver has an inner perimeter that encloses all the module's/driver's queues.

       None of the above    The module/driver has no inner perimeter.

   Outer Perimeter Flags
       The outer perimeter presence is configured using:

       D_MTOUTPERIM    In addition to any inner perimeter, the module/driver has an  outer  perimeter  that  encloses  all  the  module's/driver's
		       queues. This can be combined with all the inner perimeter options except D_MTPERMOD.

       Note  that  acquiring  exclusive access at the outer perimeter (that is, using qwriter(9F) with the PERIM_OUTER flag) can incur significant
       performance penalties, which grow linearly with the number of open instances of the module or driver in the system.

       The default concurrency can be modified using:

       D_MTPUTSHARED	This flag modifies the default behavior when put(9E) procedure are invoked so that the inner perimeter is  entered  shared
			instead of exclusively.

       D_MTOCEXCL	This  flag  modifies  the  default  behavior  when open(9E) and close(9E) procedures are invoked so the outer perimeter is
			entered exclusively instead of shared.

			Note that drivers and modules using this flag can cause significant system performance degradation during stream  open	or
			close when many instances of the driver or module are in use simultaneously. For this reason, use of this flag is discour-
			aged. Instead, since open(9E) and close(9E) both execute with user context, developers are encouraged to  use  traditional
			synchronization routines such as cv_wait_sig(9F) to coordinate with other open instances of the module or driver.

       The module/driver can use qwait(9F) or qwait_sig() in the open(9E) and close(9E) procedures if it needs to wait "outside" the perimeters.

       The module/driver can use qwriter(9F) to upgrade the access at the inner or outer perimeter from shared to exclusive.

       The use and semantics of qprocson() and qprocsoff(9F) is independent of the inner and outer perimeters.

SEE ALSO

       close(9E), open(9E), put(9E), srv(9E), qprocsoff(9F), qprocson(9F), qwait(9F), qwriter(9F), cb_ops(9S)

       STREAMS Programming Guide

       Writing Device Drivers

SunOS 5.11							    16 Dec 2002 						    mt-streams(9F)