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This change makes XRay print the log file output only when the verbosity level is higher than 0. It reduces the log spam in the default case when we want XRay running silently, except when there are actual fatal/serious errors. We also update the documentation to show how to get the information after the change to the default behaviour. llvm-svn: 320550
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338 lines
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ReStructuredText
===================
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Debugging with XRay
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===================
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This document shows an example of how you would go about analyzing applications
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built with XRay instrumentation. Here we will attempt to debug ``llc``
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compiling some sample LLVM IR generated by Clang.
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.. contents::
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:local:
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Building with XRay
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------------------
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To debug an application with XRay instrumentation, we need to build it with a
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Clang that supports the ``-fxray-instrument`` option. See `XRay <XRay.html>`_
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for more technical details of how XRay works for background information.
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In our example, we need to add ``-fxray-instrument`` to the list of flags
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passed to Clang when building a binary. Note that we need to link with Clang as
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well to get the XRay runtime linked in appropriately. For building ``llc`` with
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XRay, we do something similar below for our LLVM build:
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::
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$ mkdir -p llvm-build && cd llvm-build
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# Assume that the LLVM sources are at ../llvm
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$ cmake -GNinja ../llvm -DCMAKE_BUILD_TYPE=Release \
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-DCMAKE_C_FLAGS_RELEASE="-fxray-instrument" -DCMAKE_CXX_FLAGS="-fxray-instrument" \
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# Once this finishes, we should build llc
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$ ninja llc
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To verify that we have an XRay instrumented binary, we can use ``objdump`` to
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look for the ``xray_instr_map`` section.
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::
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$ objdump -h -j xray_instr_map ./bin/llc
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./bin/llc: file format elf64-x86-64
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Sections:
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Idx Name Size VMA LMA File off Algn
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14 xray_instr_map 00002fc0 00000000041516c6 00000000041516c6 03d516c6 2**0
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CONTENTS, ALLOC, LOAD, READONLY, DATA
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Getting Traces
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--------------
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By default, XRay does not write out the trace files or patch the application
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before main starts. If we just run ``llc`` it should just work like a normally
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built binary. However, if we want to get a full trace of the application's
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operations (of the functions we do end up instrumenting with XRay) then we need
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to enable XRay at application start. To do this, XRay checks the
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``XRAY_OPTIONS`` environment variable.
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::
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# The following doesn't create an XRay trace by default.
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$ ./bin/llc input.ll
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# We need to set the XRAY_OPTIONS to enable some features.
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$ XRAY_OPTIONS="patch_premain=true xray_mode=xray-basic verbosity=1" ./bin/llc input.ll
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==69819==XRay: Log file in 'xray-log.llc.m35qPB'
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At this point we now have an XRay trace we can start analysing.
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The ``llvm-xray`` Tool
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----------------------
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Having a trace then allows us to do basic accounting of the functions that were
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instrumented, and how much time we're spending in parts of the code. To make
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sense of this data, we use the ``llvm-xray`` tool which has a few subcommands
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to help us understand our trace.
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One of the simplest things we can do is to get an accounting of the functions
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that have been instrumented. We can see an example accounting with ``llvm-xray
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account``:
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::
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$ llvm-xray account xray-log.llc.m35qPB -top=10 -sort=sum -sortorder=dsc -instr_map ./bin/llc
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Functions with latencies: 29
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funcid count [ min, med, 90p, 99p, max] sum function
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187 360 [ 0.000000, 0.000001, 0.000014, 0.000032, 0.000075] 0.001596 LLLexer.cpp:446:0: llvm::LLLexer::LexIdentifier()
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85 130 [ 0.000000, 0.000000, 0.000018, 0.000023, 0.000156] 0.000799 X86ISelDAGToDAG.cpp:1984:0: (anonymous namespace)::X86DAGToDAGISel::Select(llvm::SDNode*)
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138 130 [ 0.000000, 0.000000, 0.000017, 0.000155, 0.000155] 0.000774 SelectionDAGISel.cpp:2963:0: llvm::SelectionDAGISel::SelectCodeCommon(llvm::SDNode*, unsigned char const*, unsigned int)
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188 103 [ 0.000000, 0.000000, 0.000003, 0.000123, 0.000214] 0.000737 LLParser.cpp:2692:0: llvm::LLParser::ParseValID(llvm::ValID&, llvm::LLParser::PerFunctionState*)
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88 1 [ 0.000562, 0.000562, 0.000562, 0.000562, 0.000562] 0.000562 X86ISelLowering.cpp:83:0: llvm::X86TargetLowering::X86TargetLowering(llvm::X86TargetMachine const&, llvm::X86Subtarget const&)
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125 102 [ 0.000001, 0.000003, 0.000010, 0.000017, 0.000049] 0.000471 Verifier.cpp:3714:0: (anonymous namespace)::Verifier::visitInstruction(llvm::Instruction&)
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90 8 [ 0.000023, 0.000035, 0.000106, 0.000106, 0.000106] 0.000342 X86ISelLowering.cpp:3363:0: llvm::X86TargetLowering::LowerCall(llvm::TargetLowering::CallLoweringInfo&, llvm::SmallVectorImpl<llvm::SDValue>&) const
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124 32 [ 0.000003, 0.000007, 0.000016, 0.000041, 0.000041] 0.000310 Verifier.cpp:1967:0: (anonymous namespace)::Verifier::visitFunction(llvm::Function const&)
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123 1 [ 0.000302, 0.000302, 0.000302, 0.000302, 0.000302] 0.000302 LLVMContextImpl.cpp:54:0: llvm::LLVMContextImpl::~LLVMContextImpl()
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139 46 [ 0.000000, 0.000002, 0.000006, 0.000008, 0.000019] 0.000138 TargetLowering.cpp:506:0: llvm::TargetLowering::SimplifyDemandedBits(llvm::SDValue, llvm::APInt const&, llvm::APInt&, llvm::APInt&, llvm::TargetLowering::TargetLoweringOpt&, unsigned int, bool) const
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This shows us that for our input file, ``llc`` spent the most cumulative time
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in the lexer (a total of 1 millisecond). If we wanted for example to work with
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this data in a spreadsheet, we can output the results as CSV using the
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``-format=csv`` option to the command for further analysis.
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If we want to get a textual representation of the raw trace we can use the
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``llvm-xray convert`` tool to get YAML output. The first few lines of that
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output for an example trace would look like the following:
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::
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$ llvm-xray convert -f yaml -symbolize -instr_map=./bin/llc xray-log.llc.m35qPB
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---
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header:
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version: 1
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type: 0
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constant-tsc: true
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nonstop-tsc: true
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cycle-frequency: 2601000000
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records:
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- { type: 0, func-id: 110, function: __cxx_global_var_init.8, cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426023268520 }
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- { type: 0, func-id: 110, function: __cxx_global_var_init.8, cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426023523052 }
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- { type: 0, func-id: 164, function: __cxx_global_var_init, cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426029925386 }
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- { type: 0, func-id: 164, function: __cxx_global_var_init, cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426030031128 }
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- { type: 0, func-id: 142, function: '(anonymous namespace)::CommandLineParser::ParseCommandLineOptions(int, char const* const*, llvm::StringRef, llvm::raw_ostream*)', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426046951388 }
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- { type: 0, func-id: 142, function: '(anonymous namespace)::CommandLineParser::ParseCommandLineOptions(int, char const* const*, llvm::StringRef, llvm::raw_ostream*)', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426047282020 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426047857332 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426047984152 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426048036584 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426048042292 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-enter, tsc: 5434426048055056 }
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- { type: 0, func-id: 187, function: 'llvm::LLLexer::LexIdentifier()', cpu: 37, thread: 69819, kind: function-exit, tsc: 5434426048067316 }
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Controlling Fidelity
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--------------------
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So far in our examples, we haven't been getting full coverage of the functions
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we have in the binary. To get that, we need to modify the compiler flags so
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that we can instrument more (if not all) the functions we have in the binary.
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We have two options for doing that, and we explore both of these below.
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Instruction Threshold
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`````````````````````
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The first "blunt" way of doing this is by setting the minimum threshold for
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function bodies to 1. We can do that with the
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``-fxray-instruction-threshold=N`` flag when building our binary. We rebuild
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``llc`` with this option and observe the results:
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::
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$ rm CMakeCache.txt
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$ cmake -GNinja ../llvm -DCMAKE_BUILD_TYPE=Release \
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-DCMAKE_C_FLAGS_RELEASE="-fxray-instrument -fxray-instruction-threshold=1" \
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-DCMAKE_CXX_FLAGS="-fxray-instrument -fxray-instruction-threshold=1"
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$ ninja llc
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$ XRAY_OPTIONS="patch_premain=true" ./bin/llc input.ll
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==69819==XRay: Log file in 'xray-log.llc.5rqxkU'
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$ llvm-xray account xray-log.llc.5rqxkU -top=10 -sort=sum -sortorder=dsc -instr_map ./bin/llc
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Functions with latencies: 36652
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funcid count [ min, med, 90p, 99p, max] sum function
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75 1 [ 0.672368, 0.672368, 0.672368, 0.672368, 0.672368] 0.672368 llc.cpp:271:0: main
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78 1 [ 0.626455, 0.626455, 0.626455, 0.626455, 0.626455] 0.626455 llc.cpp:381:0: compileModule(char**, llvm::LLVMContext&)
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139617 1 [ 0.472618, 0.472618, 0.472618, 0.472618, 0.472618] 0.472618 LegacyPassManager.cpp:1723:0: llvm::legacy::PassManager::run(llvm::Module&)
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139610 1 [ 0.472618, 0.472618, 0.472618, 0.472618, 0.472618] 0.472618 LegacyPassManager.cpp:1681:0: llvm::legacy::PassManagerImpl::run(llvm::Module&)
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139612 1 [ 0.470948, 0.470948, 0.470948, 0.470948, 0.470948] 0.470948 LegacyPassManager.cpp:1564:0: (anonymous namespace)::MPPassManager::runOnModule(llvm::Module&)
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139607 2 [ 0.147345, 0.315994, 0.315994, 0.315994, 0.315994] 0.463340 LegacyPassManager.cpp:1530:0: llvm::FPPassManager::runOnModule(llvm::Module&)
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139605 21 [ 0.000002, 0.000002, 0.102593, 0.213336, 0.213336] 0.463331 LegacyPassManager.cpp:1491:0: llvm::FPPassManager::runOnFunction(llvm::Function&)
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139563 26096 [ 0.000002, 0.000002, 0.000037, 0.000063, 0.000215] 0.225708 LegacyPassManager.cpp:1083:0: llvm::PMDataManager::findAnalysisPass(void const*, bool)
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108055 188 [ 0.000002, 0.000120, 0.001375, 0.004523, 0.062624] 0.159279 MachineFunctionPass.cpp:38:0: llvm::MachineFunctionPass::runOnFunction(llvm::Function&)
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62635 22 [ 0.000041, 0.000046, 0.000050, 0.126744, 0.126744] 0.127715 X86TargetMachine.cpp:242:0: llvm::X86TargetMachine::getSubtargetImpl(llvm::Function const&) const
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Instrumentation Attributes
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``````````````````````````
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The other way is to use configuration files for selecting which functions
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should always be instrumented by the compiler. This gives us a way of ensuring
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that certain functions are either always or never instrumented by not having to
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add the attribute to the source.
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To use this feature, you can define one file for the functions to always
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instrument, and another for functions to never instrument. The format of these
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files are exactly the same as the SanitizerLists files that control similar
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things for the sanitizer implementations. For example, we can have two
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different files like below:
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::
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# always-instrument.txt
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# always instrument functions that match the following filters:
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fun:main
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# never-instrument.txt
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# never instrument functions that match the following filters:
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fun:__cxx_*
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Given the above two files we can re-build by providing those two files as
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arguments to clang as ``-fxray-always-instrument=always-instrument.txt`` or
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``-fxray-never-instrument=never-instrument.txt``.
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The XRay stack tool
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-------------------
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Given a trace, and optionally an instrumentation map, the ``llvm-xray stack``
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command can be used to analyze a call stack graph constructed from the function
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call timeline.
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The simplest way to use the command is simply to output the top stacks by call
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count and time spent.
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::
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$ llvm-xray stack xray-log.llc.5rqxkU -instr_map ./bin/llc
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Unique Stacks: 3069
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Top 10 Stacks by leaf sum:
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Sum: 9633790
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lvl function count sum
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#0 main 1 58421550
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#1 compileModule(char**, llvm::LLVMContext&) 1 51440360
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#2 llvm::legacy::PassManagerImpl::run(llvm::Module&) 1 40535375
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#3 llvm::FPPassManager::runOnModule(llvm::Module&) 2 39337525
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#4 llvm::FPPassManager::runOnFunction(llvm::Function&) 6 39331465
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#5 llvm::PMDataManager::verifyPreservedAnalysis(llvm::Pass*) 399 16628590
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#6 llvm::PMTopLevelManager::findAnalysisPass(void const*) 4584 15155600
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#7 llvm::PMDataManager::findAnalysisPass(void const*, bool) 32088 9633790
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..etc..
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In the default mode, identical stacks on different threads are independently
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aggregated. In a multithreaded program, you may end up having identical call
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stacks fill your list of top calls.
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To address this, you may specify the ``-aggregate-threads`` or
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``-per-thread-stacks`` flags. ``-per-thread-stacks`` treats the thread id as an
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implicit root in each call stack tree, while ``-aggregate-threads`` combines
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identical stacks from all threads.
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Flame Graph Generation
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----------------------
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The ``llvm-xray stack`` tool may also be used to generate flamegraphs for
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visualizing your instrumented invocations. The tool does not generate the graphs
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themselves, but instead generates a format that can be used with Brendan Gregg's
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FlameGraph tool, currently available on `github
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<https://github.com/brendangregg/FlameGraph>`_.
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To generate output for a flamegraph, a few more options are necessary.
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- ``-all-stacks`` - Emits all of the stacks instead of just the top stacks.
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- ``-stack-format`` - Choose the flamegraph output format 'flame'.
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- ``-aggregation-type`` - Choose the metric to graph.
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You may pipe the command output directly to the flamegraph tool to obtain an
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svg file.
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::
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$llvm-xray stack xray-log.llc.5rqxkU -instr_map ./bin/llc -stack-format=flame -aggregation-type=time -all-stacks | \
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/path/to/FlameGraph/flamegraph.pl > flamegraph.svg
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If you open the svg in a browser, mouse events allow exploring the call stacks.
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Further Exploration
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-------------------
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The ``llvm-xray`` tool has a few other subcommands that are in various stages
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of being developed. One interesting subcommand that can highlight a few
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interesting things is the ``graph`` subcommand. Given for example the following
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toy program that we build with XRay instrumentation, we can see how the
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generated graph may be a helpful indicator of where time is being spent for the
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application.
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.. code-block:: c++
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// sample.cc
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#include <iostream>
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#include <thread>
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[[clang::xray_always_instrument]] void f() {
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std::cerr << '.';
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}
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[[clang::xray_always_instrument]] void g() {
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for (int i = 0; i < 1 << 10; ++i) {
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std::cerr << '-';
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}
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}
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int main(int argc, char* argv[]) {
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std::thread t1([] {
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for (int i = 0; i < 1 << 10; ++i)
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f();
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});
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std::thread t2([] {
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g();
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});
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t1.join();
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t2.join();
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std::cerr << '\n';
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}
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We then build the above with XRay instrumentation:
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::
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$ clang++ -o sample -O3 sample.cc -std=c++11 -fxray-instrument -fxray-instruction-threshold=1
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$ XRAY_OPTIONS="patch_premain=true" ./sample
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We can then explore the graph rendering of the trace generated by this sample
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application. We assume you have the graphviz toosl available in your system,
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including both ``unflatten`` and ``dot``. If you prefer rendering or exploring
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the graph using another tool, then that should be feasible as well. ``llvm-xray
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graph`` will create DOT format graphs which should be usable in most graph
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rendering applications. One example invocation of the ``llvm-xray graph``
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command should yield some interesting insights to the workings of C++
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applications:
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::
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$ llvm-xray graph xray-log.sample.* -m sample -color-edges=sum -edge-label=sum \
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| unflatten -f -l10 | dot -Tsvg -o sample.svg
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Next Steps
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----------
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If you have some interesting analyses you'd like to implement as part of the
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llvm-xray tool, please feel free to propose them on the llvm-dev@ mailing list.
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The following are some ideas to inspire you in getting involved and potentially
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making things better.
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- Implement a query/filtering library that allows for finding patterns in the
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XRay traces.
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- A conversion from the XRay trace onto something that can be visualised
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better by other tools (like the Chrome trace viewer for example).
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- Collecting function call stacks and how often they're encountered in the
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XRay trace.
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