To figure out performance problem start with making sure you use up to date drivers (WinOF 4.80 released today) as well as switch and HCA firmware (see release notes for recommended HCA fw - http://www.mellanox.com/related-docs/prod_software/MLNX_VPI_WinOF_Release_Notes_Rev_4.80.pdf). Then to rule out fabric problems like flaky cables causing unstable links and/or bad hardware check performance between hosts using IB utilities for Windows (i.e. nd_write_bw, nd_read_bw - see http://www.mellanox.com/related-docs/prod_software/MLNX_VPI_WinOF_User_Manual_v4.80.pdf). Note that with GD4036 switches your max data rate per link is 32 Gb/s with since QDR uses 8/10 encoding (40Gb/s on the wire gets you only 32Gb/s for data).
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