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Performance seems not stable after using AES-NI for data encryption/decryption

Hi, everyone.

I've got a need to encrypt data written from a virtual machine on XenServer. I added a pure software AES CBC encryption method to the Xen virtual disk read/write operation, and test the write throughput by runing the following command in the VM:

dd if=/dev/zero of=/mnt/test_file bs=512 count=1048576

and the tested throughput is about: 55 MB/s.

I modified the encryption method to use the Intel AES-NI for encryption/decrytion, and run the former test several times, and the result is as follows:

Test 1: 85.1 MB/s

Test 2: 72.0 MB/s

Test 3: 56.0 MB/s

Test 4: 95.9 MB/s

Test 5: 43.5 MB/s

Test 6: 61.5 MB/s

Test 7: 74.5 MB/s

Test 8: 43.3 MB/s

Test 9: 63.8 MB/s

Test 10: 94.8 MB/s

Test 11: 110 MB/s

Although the average throughput is about 40% higher than that using the pure software method, the throughput seems to be very unstable.

Why? Is there anyway to stablize it? Thank you:-)


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