Browse Prior Art Database

Channel Subsystem Timer

IP.com Disclosure Number: IPCOM000101085D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 9 page(s) / 338K

Publishing Venue

IBM

Related People

Cheney, D: AUTHOR [+2]

Abstract

Access to the Time of Day (TOD) clock creates a performance degradation for channel processors in XA mode since the TOD is usually in one central place, and numerous accesses are required in channel microcode for the Channel Monitoring Facility and for time stamping elements of trace arrays. These accesses are time-consuming and burdensome to time critical operations. A localized timer is needed in the channel subsystem to provide faster access.

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This is the abbreviated version, containing approximately 22% of the total text.

Channel Subsystem Timer

       Access to the Time of Day (TOD) clock creates a
performance degradation for channel processors in XA mode since the
TOD is usually in one central place, and numerous accesses are
required in channel microcode for the Channel Monitoring Facility and
for time stamping elements of trace arrays. These accesses are
time-consuming and burdensome to time critical operations.  A
localized timer is needed in the channel subsystem to provide faster
access.

      It is also desirable from a serviceability point of view to be
able to review trace array logouts from separate channel engines and
to be able to correlate events between them and the TOD clock.  This
is especially needed in an XA environment where dynamic pathing and
multi- pathing exist. Localized timers would be best suited to assist
in the task of time stamping microcode trace entries and would be
required for time stamping hardware traces where the TOD would not be
accessible.

      The requirements for such a channel subsystem timer are:
 1.   Maintain synchronism throughout the channel subsystem.
 2.   Maintain a synchronous relationship of all localized timers to
the TOD clock.
      3.   Provide flexibility for the addition of more channels or
I/O processors.

      The Channel subsystem timing facility described provides all of
the above, requiring channel microcode to access the TOD only during
system initialization or after the TOD is altered.

      The ability of one I/O processor (IOP) to establish a reference
to the Time of Day clock is an essential function of the channel
microcode.  Once a fixed reference is known, both architected and
tracing functions need only to access the local Channel timer rather
than waste valuable cycles making an external access for the central
TOD clock. The savings can be significant when considering the number
of accesses of the TOD that are required for a single Start
Subchannel operation (SSCH) and also the comparative lengths of time
it takes to do a TOD fetch versus a local timer fetch.  A fetch from
the TOD may require many cycles, perhaps on the order of 50 or
greater, since an external mechanism needs to be used.  Other
processors or IOPs in the system may be contending for either the TOD
itself or the bus/communication facility that is used to access it,
extending the latency even more.  In comparison, microcode access of
a channel timer, designated as a local register, will take just a few
cycles.

      Many of the architected timing measurements in 370/XA require
only relative times without reference to the TOD. This includes
device connect time and device disconnect time.  Other measurements
do need a reference to the TOD, such as function pending time or
internally maintained microcode traces.  Traditional methods of
keeping track of these times by using the TOD clock add an ever
increasing burden to high performance channel subsystems.  A partial
solutio...