Answers Database

Long Runtimes for Some Designs


Record #1869

Problem Title:
Long Runtimes for Some Designs


Problem Description:
Platform(s): All
Architecture(s): All
Design Step(s): TRCE, PAR
Reference Number: 13236
XACT Performance may require very long runtimes to analyze some
designs.


Solution 1:

Workaround: Runtime can now be controlled using the
XILINX_PATHLIMIT environment variable, which limits the number
of connection evaluations per timing constraint to the specified number.  Settin
g the XILINX_PATHLIMIT variable to a low number reduces the number of paths anal
yzed, and consequently reduces the runtime needed to run TRCE, ITA_FPGA, and PAR
. When using this environment variable, a message is displayed indicating that a
nalysis has been truncated. Following is a sample message:

WARNING:0 - The current connection evaluation limit of
1000 caused the truncation of timing analysis for
paths through 2.88% of constrained connections, which
may limit the accuracy of this analysis. You can
specify a larger limit with the XILINX_PATHLIMIT
environment variable to increase the accuracy of this
analysis.

This message indicates the percentage of design connections where analysis was t
runcated in order to reduce overall runtime. If this message appears, analysis i
s incomplete and may not guarantee correct timing results. You are encouraged to
 use the largest limit possible that provides an acceptable runtime in order to
ensure the most accurate timing analysis possible.The timing report also contain
s a list of design connections where analysis was truncated, and you should chec
k these against the reported critical path(s) to help determine if irrelevant pa
ths are being analyzed by the tools.  In some cases, long runtimes may result fr
om large groupings used in the FROM:TO type of TIMESPECs. In these cases, runtim
es can be improved in these ways:

Specify a PERIOD TIMESPEC that includes the majority of the
paths to be analyzed (for example, TIMEGRP "ALL_FFS"
PERIOD = 30), and use the FROM:TO TIMESPECs only for smaller groups of multicycl
e paths not included in the global PERIOD TIMESPEC.

When possible, use a MAXDELAY TO: constraint instead of a
MAXDELAY TO:FROM constraint.



End of Record #1869