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authorBruce Richardson <bruce.richardson@intel.com>2017-08-04 13:44:11 +0100
committerThomas Monjalon <thomas@monjalon.net>2017-08-06 12:34:07 +0200
commit1f4c80df39c48095064aa3c44863a46297463ac6 (patch)
tree62aa886a0d63265a5206b5f34c2875efb91aaec3
parent5dd667e15ba8816dbb7e8dfcb727ac09c8a2af83 (diff)
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doc: update FAQ with new testpmd default burst size
Fixes: 836853d3d4cf ("app/testpmd: increase default burst size to 32") Signed-off-by: Bruce Richardson <bruce.richardson@intel.com> Acked-by: John McNamara <john.mcnamara@intel.com>
-rw-r--r--doc/guides/faq/faq.rst14
1 files changed, 7 insertions, 7 deletions
diff --git a/doc/guides/faq/faq.rst b/doc/guides/faq/faq.rst
index f21aff4..dac8050 100644
--- a/doc/guides/faq/faq.rst
+++ b/doc/guides/faq/faq.rst
@@ -115,16 +115,16 @@ but the end-to-end latency of an average packet typically increases as a result.
Similarly, the application can be tuned to have, on average, a low end-to-end latency at the cost of lower throughput.
To achieve higher throughput, the DPDK attempts to aggregate the cost of processing each packet individually by processing packets in bursts.
-Using the testpmd application as an example, the "burst" size can be set on the command line to a value of 16 (also the default value).
-This allows the application to request 16 packets at a time from the PMD.
-The testpmd application then immediately attempts to transmit all the packets that were received, in this case, all 16 packets.
+Using the testpmd application as an example, the "burst" size can be set on the command line to a value of 32 (also the default value).
+This allows the application to request 32 packets at a time from the PMD.
+The testpmd application then immediately attempts to transmit all the packets that were received, in this case, all 32 packets.
The packets are not transmitted until the tail pointer is updated on the corresponding TX queue of the network port.
This behavior is desirable when tuning for high throughput because the cost of tail pointer updates to both the RX and TX queues
-can be spread across 16 packets, effectively hiding the relatively slow MMIO cost of writing to the PCIe* device.
+can be spread across 32 packets, effectively hiding the relatively slow MMIO cost of writing to the PCIe* device.
-However, this is not very desirable when tuning for low latency, because the first packet that was received must also wait for the other 15 packets to be received.
-It cannot be transmitted until the other 15 packets have also been processed because the NIC will not know to transmit the packets until the TX tail pointer has been updated,
-which is not done until all 16 packets have been processed for transmission.
+However, this is not very desirable when tuning for low latency, because the first packet that was received must also wait for the other 31 packets to be received.
+It cannot be transmitted until the other 31 packets have also been processed because the NIC will not know to transmit the packets until the TX tail pointer has been updated,
+which is not done until all 32 packets have been processed for transmission.
To consistently achieve low latency even under heavy system load, the application developer should avoid processing packets in bunches.
The testpmd application can be configured from the command line to use a burst value of 1.