While profiling my kernels in Visual Profiler on Kepler hardware, I’ve noticed the profiler shows that global loads and stores are cached in L1. I'm confused because the programming guide and Kepler tuning manual state that:
L1 caching in Kepler GPUs is reserved only for local memory accesses, such as register spills and stack data. Global loads are cached in L2 only (or in the Read-Only Data Cache).
There are no register spills (profiler shows L1 caching even for primitive, 2-lines 'add' kernel) and I'm not sure what 'stack data' means here.
GK110 Whitepaper shows that global accesses will go through L1 cache in all but one case: loads through read-only cache (__ldg). Does it mean that while global accesses go through L1 hardware they are not actually cached? Does it also mean that if I have spilled registers data cached in L1, this data can be evicted as a result of gmem access?
UPDATE: I've realized that I might be misreading the information the profiler is giving to me, so here is the kernel code as well as profiler results (I've tried both on Titan and K40 with the same results).
template<typename T>
__global__ void addKernel(T *c, const T *a, const T *b)
{
int i = blockIdx.x * blockDim.x + threadIdx.x;
c[i] = a[i] + b[i];
}
...
// Kernel call
float* x;
float* y;
float* d;
// ...
addKernel<<<1024, 1024>>>(d, x, y);
cudaError_t cudaStatus = cudaDeviceSynchronize();
assert(cudaSuccess == cudaStatus);
Visual Profiler output:
L1 numbers make perfect sense given L1 cache is enabled for gmem accesses. For the loads we have:
65536 * 128 == 2 * 4 * 1024 * 1024
UPDATE 2: added SASS and PTX code. SASS code is very simple and contains reads from constant memory and loads/stores from/to global memory (LD/ST instructions).
Function : _Z9addKernelIfEvPT_PKS0_S3_
.headerflags @"EF_CUDA_SM35 EF_CUDA_PTX_SM(EF_CUDA_SM35)"
/* 0x088cb0a0a08c1000 */
/*0008*/ MOV R1, c[0x0][0x44]; /* 0x64c03c00089c0006 */
/*0010*/ S2R R0, SR_CTAID.X; /* 0x86400000129c0002 */
/*0018*/ MOV32I R5, 0x4; /* 0x74000000021fc016 */
/*0020*/ S2R R3, SR_TID.X; /* 0x86400000109c000e */
/*0028*/ IMAD R2, R0, c[0x0][0x28], R3; /* 0x51080c00051c000a */
/*0030*/ IMAD R6.CC, R2, R5, c[0x0][0x148]; /* 0x910c1400291c081a */
/*0038*/ IMAD.HI.X R7, R2, R5, c[0x0][0x14c]; /* 0x93181400299c081e */
/* 0x08a0a4b0809c80b0 */
/*0048*/ IMAD R8.CC, R2, R5, c[0x0][0x150]; /* 0x910c14002a1c0822 */
/*0050*/ IMAD.HI.X R9, R2, R5, c[0x0][0x154]; /* 0x931814002a9c0826 */
/*0058*/ LD.E R3, [R6]; /* 0xc4800000001c180c */
/*0060*/ LD.E R0, [R8]; /* 0xc4800000001c2000 */
/*0068*/ IMAD R4.CC, R2, R5, c[0x0][0x140]; /* 0x910c1400281c0812 */
/*0070*/ IMAD.HI.X R5, R2, R5, c[0x0][0x144]; /* 0x93181400289c0816 */
/*0078*/ FADD R0, R3, R0; /* 0xe2c00000001c0c02 */
/* 0x080000000000b810 */
/*0088*/ ST.E [R4], R0; /* 0xe4800000001c1000 */
/*0090*/ EXIT ; /* 0x18000000001c003c */
/*0098*/ BRA 0x98; /* 0x12007ffffc1c003c */
/*00a0*/ NOP; /* 0x85800000001c3c02 */
/*00a8*/ NOP; /* 0x85800000001c3c02 */
/*00b0*/ NOP; /* 0x85800000001c3c02 */
/*00b8*/ NOP; /* 0x85800000001c3c02 */
PTX:
.visible .entry _Z9addKernelIfEvPT_PKS0_S3_(
.param .u64 _Z9addKernelIfEvPT_PKS0_S3__param_0,
.param .u64 _Z9addKernelIfEvPT_PKS0_S3__param_1,
.param .u64 _Z9addKernelIfEvPT_PKS0_S3__param_2
)
{
.reg .s32 %r<5>;
.reg .f32 %f<4>;
.reg .s64 %rd<11>;
ld.param.u64 %rd1, [_Z9addKernelIfEvPT_PKS0_S3__param_0];
ld.param.u64 %rd2, [_Z9addKernelIfEvPT_PKS0_S3__param_1];
ld.param.u64 %rd3, [_Z9addKernelIfEvPT_PKS0_S3__param_2];
cvta.to.global.u64 %rd4, %rd1;
.loc 1 22 1
mov.u32 %r1, %ntid.x;
mov.u32 %r2, %ctaid.x;
mov.u32 %r3, %tid.x;
mad.lo.s32 %r4, %r1, %r2, %r3;
cvta.to.global.u64 %rd5, %rd2;
mul.wide.s32 %rd6, %r4, 4;
add.s64 %rd7, %rd5, %rd6;
cvta.to.global.u64 %rd8, %rd3;
add.s64 %rd9, %rd8, %rd6;
.loc 1 23 1
ld.global.f32 %f1, [%rd9];
ld.global.f32 %f2, [%rd7];
add.f32 %f3, %f2, %f1;
add.s64 %rd10, %rd4, %rd6;
.loc 1 23 1
st.global.f32 [%rd10], %f3;
.loc 1 24 2
ret;
}
