NAME

mt-streams - STREAMS multithreading

SYNOPSIS

#include <sys/conf.h>

INTERFACE LEVEL

illumos DDI specific (illumos DDI).

DESCRIPTION

STREAMS drivers configure 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 perimeter (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 discouraged. 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.