Runtime: Linux stack walk takes 23x the time as Windows

Created on 31 Jan 2019 · 6Comments · Source: dotnet/runtime

Repro steps

Create a new console app and paste in the following code
Open the folder in VS Code
Set a breakpoint on the call to 'Nop' with a condition of 'x == 100'
Launch the program under the debugger and wait for it to exit

Result:

This is much slower on Linux than Windows. By adding QueryPerformanceCounter calls, I found that the difference is that stack walk is 23x on Linux than Windows (160.5 ms vs. 6.8 ms per walk).

Notes

More instrumentation would be needed to diagnose exactly what part of stack walk is slow, but here where the CPU is active in this scenario:

NOTE: This is an inverted call stack that originally had mangled symbol names. I partially unmangled the names by hand.

libvsdebugeng.so!dispatcher::DkmThread::GetTopStackWalkFrame(DkmRuntimeInstance, DkmStackWalkFrame)
libvsdebugeng.impl.so!MonitorStackMerge::CMergeObj::GetTopStackWalkFrame(DkmThread, DkmRuntimeInstance, CallStack::DkmStackWalkFrame)
libvsdebugeng.impl.so!Common::GetRegistersForThread(DkmThread, DkmRuntimeInstance, DkmFrameRegisters)
libvsdebugeng.impl.so!Common::GetTopStackPointerForThread(DkmThread)
libvsdebugeng.impl.so!Microsoft::VisualStudio::Debugger::DkmThread::GetContext()
libvsdebugeng.so!Proc6C71BAFC0CA8B6BB34725A9DB87FBDAC
libvsdebugeng.so!dispatcher::DkmThread::GetContext()
libvsdebugeng.impl.so!ManagedDM::CV2EntryPoint::GetContext(DkmThread)
libmscordbi.so!CordbThread::GetRegisterSet(ICorDebugRegisterSet)
libmscordbi.so!ShimProcess::LookupOrCreateShimStackWalk(ICorDebugThread)
libmscordbi.so!ShimStackWalk::ShimProcess(CorDebugThread)
libmscordbi.so!ShimStackWalk::Populate()
libmscordbi.so!CordbThread::CreateStackWalk(ICorDebugStackWalk)
libmscordbi.so!CordbStackWalk::Init()
libmscordaccore.so!DacDbiInterfaceImpl::CreateStackWalk(VMPTR_BaseI6Thread6__VPtrIS1_EEP10DT_CONTEXTPPP)
libmscordaccore.so!DacDbiInterfaceImp::SetStackWalkCurrentContext(VMPTR_BaseI6Thread6__VPtrIS1_EEPPv22CorDebugSetContextFlagP10DT_CONTEXT)
libmscordaccore.so!StackFrameIterator::ResetRegDisp(REGDISPLAY)
libmscordaccore.so!EECodeManager::EnsureCallerContextIsValid(PREGDISPLAY, StackwalkCacheEntry pCacheEntry, EECodeInfo * pCodeInfo)
libmscordaccore.so!Thread::VirtualUnwindCallFram(CONTEXTP30_KNONVOLATILE_CONTEXT_POINTERSP10EECodeInfo)
libmscordaccore.so!DacUnwindStackFrame(T_CONTEXTP30_KNONVOLATILE_CONTEXT_POINTERS
libmscordaccore.so!StackUnwinderAMD6413VirtualUnwindEjmmP29_IMAGE_RUNTIME_FUNCTION_ENTRYP8_CONTEXTPPvPmP30_KNONVOLATILE_CONTEXT_POINTERSPPFiP17_EXCEPTION_RECORDmS3_S4_E
libmscordaccore.so!DacReplacePatchesInHostMemory
libmscordaccore.so!CHashTable13FindNextEntryEP8HASHFIND
libmscordaccore.so!DacInstantiateTypeByAddressHelpermjbb
libmscordbi.so!ShimRemoteDataTarget11ReadVirtualEmPhjPj
libmscordbi.so!DbgTransportSession10ReadMemoryEPhS0_m
libmscordbi.so!DbgTransportSession25SendRequestMessageAndWaitEPNS_7MessageE

Program

```C#
using System;
using System.Diagnostics;

namespace HitCountBpTest22
{
class Program
{
static void Main(string[] args)
{
int ms = HitCountFunc();
Console.WriteLine("HitCountFunc executed in {0} milliseconds.", ms);
}

    static int HitCountFunc()
    {
        Stopwatch sw = new Stopwatch();
        sw.Start();

        for (int x = 0; x < 90; x++)
        {
            Nop(x); // Set breakpoint here with condition 'x == 100'
        }

        sw.Stop();

        return (int)sw.ElapsedMilliseconds;
    }

    static void Nop(int x)
    {            
    }
}

}
```

area-Diagnostics-coreclr tenet-performance

Source