CODEWITHSUNDEEP
openclgpgpuhpc
pradyot
pradyot

Reputation: 174

Memory usage in Dual GPU(Multi GPU)

I am using two GPUs of same configuration for my HPC GPGPU calculation using OpenCL. One of the card is connected for the display purpose and about 200-300 MB of memory is always used by two programs called compiz and x server.

My question is , when using these GPU's for computation I can use only a partial amount of total memory in GPU which is used for display purpose whereas the 2nd GPU I am able to use entire Global memory.

In my case I am using two Nvidia Quadro 410, Which has 192 cuda cores , 512 MB as memory but 503 MB usable . In case of display GPU i can use only 128MB for computation and other I can use full 503 MB for calculation.

According to the The OpenCL Specification Page 32

Max size of memory obj ect allocation in bytes. The minimum value is max (1/4 th of CL_DEVICE_GLOBAL_MEM_SIZE , 128*1024*1024)

Also shouldn't this hold good for all the GPU's present in the System?

Upvotes: 0

Views: 522

Answers (2)

DarkZeros
DarkZeros

Reputation: 8410

OpenCL will automatically swap data out or it the GPU, so you are not actually limited to the GPU global memory, you can have more memory used, as long as you don use it all at once. You can "obviusly" not create objects such big that don fit on the GPU memory. That is where this limit kicks in.

The current max limit per object is as pointed out by @huseyin

CL_DEVICE_MAX_MEM_ALLOC_SIZE (cl_ulong) Max size of memory object allocation in bytes. The minimum value is max (1/4 th of CL_DEVICE_GLOBAL_MEM_SIZE, 128*1024*1024)

minimun_max_global_size = max(1/4*global, 128MB)

If you read it carefuly, is the min value for the max allocation size.(tricky wording!). Probably nVIDIA is setting it to 1/4 on a display GPU, and to whole memory size on the non-display GPU. But nVIDIA is following the spec by doing so in both cases.

It is something you should query, and operate within the limits the API reports. You cannot change it, and you should not guess it.

Upvotes: 1

huseyin tugrul buyukisik
huseyin tugrul buyukisik

Reputation: 11926

Just continue to read from that point, you will see

Max size of memory object allocation
in bytes. The minimum value is max
(1/4th of
CL_DEVICE_GLOBAL_MEM_SIZE ,
128*1024*1024)

so whichever is greater, 128MB or 1/4 of total; will be the limit.

Upvotes: 2

Related Questions