what does gld_request really mean?

Accelerated Computing CUDA CUDA Programming and Performance
1

I am trying to profile my program with cuda profiler. I used the following events:

gld_32b : 32-byte global memory load transactions
gld_64b : 64-byte global memory load transactions
gld_128b : 128-byte global memory load transactions
gld_request : Global memory loads

My understanding is that gld_request=gld_32b+gld_64b+gld_128b.
But I am getting this output:

gld_32b=[ 9315200 ]
gld_64b=[ 3031600 ]
gld_128b=[ 1537150 ]
gld_request=[ 1962736 ]

So what does gld_request really mean?

I am using GTX280 on Redhat EL_5.3_x86.

2

I don’t think that is correct. On the GT200, a global load request is a half-warp wide request for a global memory load from the memory controller. The GT200 memory controller can decompose the request into a sequence of transactions to service the gld_request, so one gld_request can produce more than one 32 byte, 64 byte or128 byte load (see for example Figure 5-4 in the CUDA 2.3 programming guide). So I wouldn’t expect that relationship you suggest would be valid in any case except perhaps code with perfectly coalesced read behaviour.

3

avidday, thank you for you reply.

Now the data can be explained.

But I still have a set of odd data now here:

gst_32b=1638400

gst_64b=0

gst_128b=307200

gst_request=57344

Summing all together, the number of memory transactions is 1945600. So each request generates 1945600/57344=33.9 transactions. :-(

Am I getting something wrong here?

4

By the way, I am using the atomicCAS and atomicAdd operations heavily. Could that have caused the problem?

5

That looks somewhat suspicious and incorrect, but still hints that you have nearly completly random global memory writes and it asks for optimisation :)

6

Well, I tried my best to make the memory access pattern more coalesce-able. This is totally beyond my imagination.

I would accept the reality if transaction/request = 10, but not 33. External Media