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AR# 9681

3.1i Virtex-E PAR - Use of USELOWSKEWLINES impacts runtime and QOR


Keywords: Timing, Score, Runtime, USELOWSKEWLINES, QOR

Urgency: Standard

General Description:
A case has been seen where a XCV2000E design runs through PAR
quickly (45 min) and with good QOR (met timing on first route iteration); however,
if the following constraints are used, the runtime and QOR degrade


Placer score: 486086 --> 896512
Placer time: 25 min --> 2 hrs
Timing score after 1 iter: 0 --> 6603300
Total run time: 45 min --> days


When a signal with the USELOWSKEWLINES attribute is detected,
PAR automatically applies a "soft" range constraint to attempt to limit
the number of columns spanned by the loads of the signals. In this
design, the objects to which PAR was going to apply range constraints
already have range constraints that have been set by the user. In this
case, PAR should honor the user's constraints and forego introducing
additional constraints.

Aside from the placer issue mentioned above, it was also found that
routing performance was impacted by the use of the USELOWSKEWLINES
constraint. This was corrected when a MAXSKEW constraint was also
applied to these signals.

The placer problem is scheduled to be fixed in the first major release
following version 3.1i.
AR# 9681
日期 10/21/2008
状态 Archive
Type 综合文章