doc: add limitations section to cryptoperf guide

Add limitations to use of the dpdk-test-crypto-perf
tool for hardware accelerator measurements

Signed-off-by: Fiona Trahe <fiona.trahe@intel.com>
Acked-by: Slawomir Mrozowicz <slawomirx.mrozowicz@intel.com>

diff --git a/doc/guides/tools/cryptoperf.rst b/doc/guides/tools/cryptoperf.rst
index 6832312..1fc40c4 100644 (file)
--- a/doc/guides/tools/cryptoperf.rst
+++ b/doc/guides/tools/cryptoperf.rst
@@ -41,6 +41,21 @@ chain mode have to be specified in the command line as application
 parameters. These parameters are checked using device capabilities
 structure.
 
+Limitations
+-----------
+On hardware devices the cycle-count doesn't always represent the actual offload
+cost. The cycle-count only represents the offload cost when the hardware
+accelerator is not fully loaded, when loaded the cpu cycles freed up by the
+offload are still consumed by the test tool and included in the cycle-count.
+These cycles are consumed by retries and inefficient API calls enqueuing and
+dequeuing smaller bursts than specified by the cmdline parameter. This results
+in a larger cycle-count measurement and should not be interpreted as an offload
+cost measurement.
+
+On hardware devices the throughput measurement is not necessarily the maximum
+possible for the device, e.g. it may be necessary to use multiple cores to keep
+the hardware accelerator fully loaded and so measure maximum throughput.
+
 Compiling the Application
 -------------------------