NCCL 2.27.5-1

Improvements for GB200 systems
* Optimize the network performance by alternating the direction of the
  rings and the NIC to GPU assignment across communicators to limit
  unnecessary sharing.
* Fix the detection of C2C links in case GPU Direct RDMA is disabled
  between a GPU and a NIC.
* Fix PXN support on MNNVL systems, where NCCL would try (and fail) to
  share regular host memory across multiple nodes.
* Fix P2C (PXN over C2C), which is now preferred over regular PXN.  This
  support is currently preliminary and is disabled by default; use
  NCCL_PXN_C2C=1 to enable.

Further reduce the overheads of CUDA graph capturing, which increased in
NCCL 2.26.2 for large graphs.

Optimize the network performance on DGX B200 systems by adjusting the
bandwidths provided to the graph search algorithm.

Enable fp8 reductions in symmetric kernels on Blackwell with CUDA 12.8.

Restore the plugin name handling logic to make it possible to specify a
path to the plugin (Issue #1732).

Restore the ability to change NCCL_COLLNET_ENABLE during execution
(Issue #1741).

Add an example tuner plugin with CSV-based overrides.

Remove an x86 dependency from the example profiler.

ext-profiler/example/plugin.c

@@ -12,7 +12,7 @@
 #include <sys/types.h>
 #include <sys/syscall.h>
 #include <unistd.h>
-#include <x86intrin.h>
+#include <time.h>
 #include "event.h"
 #include "print_event.h"
 
@@ -41,22 +41,10 @@ static struct proxyOp* detachPool;
 ncclDebugLogger_t logFn;
 #define INFO(FLAGS, ...) logFn(NCCL_LOG_INFO, (FLAGS), __func__, __LINE__, __VA_ARGS__)
 
-static double freq = -1;
-__hidden void calibrate() {
-  struct timeval tv;
-  gettimeofday(&tv, NULL);
-  uint64_t timeCycles = __rdtsc();
-  double time = - tv.tv_sec*1e6 - tv.tv_usec;
-  uint64_t total = 0ULL;
-  for (int i = 0; i < 10000; i++) total += __rdtsc();
-  gettimeofday(&tv, NULL);
-  timeCycles = __rdtsc() - timeCycles;
-  time += tv.tv_sec*1e6 + tv.tv_usec;
-  freq = timeCycles / time;
-}
-
 __hidden double gettime(void) {
-  return __rdtsc() / freq;
+  struct timespec t;
+  clock_gettime(CLOCK_MONOTONIC, &t);
+  return (t.tv_sec*1e6 + (t.tv_nsec*1e-3));
 }
 
 static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
@@ -98,8 +86,6 @@ __hidden ncclResult_t exampleProfilerInit(void** context, int* eActivationMask,
     // process address space.
     pid = getpid();
 
-    // calibrate and start timer
-    calibrate();
     startTime = gettime();
   }
   pthread_mutex_unlock(&lock);