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Diffstat (limited to 'llvm/docs/CommandGuide/llvm-mca.rst')
| -rw-r--r-- | llvm/docs/CommandGuide/llvm-mca.rst | 53 |
1 files changed, 33 insertions, 20 deletions
diff --git a/llvm/docs/CommandGuide/llvm-mca.rst b/llvm/docs/CommandGuide/llvm-mca.rst index 5dcd97fb113..43e64c329c9 100644 --- a/llvm/docs/CommandGuide/llvm-mca.rst +++ b/llvm/docs/CommandGuide/llvm-mca.rst @@ -238,7 +238,10 @@ the following command using the example located at Iterations: 300 Instructions: 900 Total Cycles: 610 + Total uOps: 900 + Dispatch Width: 2 + uOps Per Cycle: 1.48 IPC: 1.48 Block RThroughput: 2.0 @@ -285,35 +288,45 @@ the following command using the example located at - - - 1.00 - 1.00 - - - - - - - - vhaddps %xmm3, %xmm3, %xmm4 According to this report, the dot-product kernel has been executed 300 times, -for a total of 900 dynamically executed instructions. +for a total of 900 simulated instructions. The total number of simulated micro +opcodes (uOps) is also 900. The report is structured in three main sections. The first section collects a few performance numbers; the goal of this section is to give a very quick -overview of the performance throughput. In this example, the two important -performance indicators are **IPC** and **Block RThroughput** (Block Reciprocal +overview of the performance throughput. Important performance indicators are +**IPC**, **uOps Per Cycle**, and **Block RThroughput** (Block Reciprocal Throughput). IPC is computed dividing the total number of simulated instructions by the total -number of cycles. A delta between Dispatch Width and IPC is an indicator of a -performance issue. In the absence of loop-carried data dependencies, the +number of cycles. In the absence of loop-carried data dependencies, the observed IPC tends to a theoretical maximum which can be computed by dividing the number of instructions of a single iteration by the *Block RThroughput*. -IPC is bounded from above by the dispatch width. That is because the dispatch -width limits the maximum size of a dispatch group. IPC is also limited by the -amount of hardware parallelism. The availability of hardware resources affects -the resource pressure distribution, and it limits the number of instructions -that can be executed in parallel every cycle. A delta between Dispatch -Width and the theoretical maximum IPC is an indicator of a performance -bottleneck caused by the lack of hardware resources. In general, the lower the -Block RThroughput, the better. - -In this example, ``Instructions per iteration/Block RThroughput`` is 1.50. Since -there are no loop-carried dependencies, the observed IPC is expected to approach -1.50 when the number of iterations tends to infinity. The delta between the -Dispatch Width (2.00), and the theoretical maximum IPC (1.50) is an indicator of -a performance bottleneck caused by the lack of hardware resources, and the -*Resource pressure view* can help to identify the problematic resource usage. +Field 'uOps Per Cycle' is computed dividing the total number of simulated micro +opcodes by the total number of cycles. A delta between Dispatch Width and this +field is an indicator of a performance issue. In the absence of loop-carried +data dependencies, the observed 'uOps Per Cycle' should tend to a theoretical +maximum throughput which can be computed by dividing the number of uOps of a +single iteration by the *Block RThroughput*. + +Field *uOps Per Cycle* is bounded from above by the dispatch width. That is +because the dispatch width limits the maximum size of a dispatch group. Both IPC +and 'uOps Per Cycle' are limited by the amount of hardware parallelism. The +availability of hardware resources affects the resource pressure distribution, +and it limits the number of instructions that can be executed in parallel every +cycle. A delta between Dispatch Width and the theoretical maximum uOps per +Cycle (computed by dividing the number of uOps of a single iteration by the +*Block RTrhoughput*) is an indicator of a performance bottleneck caused by the +lack of hardware resources. +In general, the lower the Block RThroughput, the better. + +In this example, ``uOps per iteration/Block RThroughput`` is 1.50. Since there +are no loop-carried dependencies, the observed *uOps Per Cycle* is expected to +approach 1.50 when the number of iterations tends to infinity. The delta between +the Dispatch Width (2.00), and the theoretical maximum uOp throughput (1.50) is +an indicator of a performance bottleneck caused by the lack of hardware +resources, and the *Resource pressure view* can help to identify the problematic +resource usage. The second section of the report shows the latency and reciprocal throughput of every instruction in the sequence. That section also reports |
