Question about running time

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Question about running time

Postby simon_zhao » Mon Aug 01, 2011 9:24 pm

I have a question about running time.
I use the routine ZHEEVX to get the largest eigenvalue and its corresponding eigenvector of a hermite matrix with (1000,1000) dimension.
The parameter LWORK is required to larger or equal to 2 * N ( which is 2000 ).
I set LWORK = 2000, and get the outcome using about 7.1 seconds.
Then by the document , I change LWORK to -1 , and get the value for LWORK from WORK(1), which is 33000.
However , I get the outcome using about 7.9 seconds.
Beacause of block algorithm , it is supposed that time using LWORK = 33000( from WORK(1) ) is shorter than time using LWORK = 2000(the least acceptable value for LWORK).
In fact , I get the opposite outcome about time.
My computer can run lapack routine normally.
Please help me and give me the reason. Thank you !
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