Messages in this thread |  | | Date | Fri, 9 Nov 2001 10:48:58 +0000 (GMT) | From | Matthew Clark <> | Subject | dev driver / pci throughput |
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Dear kernel-list,
I am writing a dev driver in which large amounts of data are passed from user space to PCI device memory and I am seeing a far lower throughput than I expected. I know this is likely to be high architecture dependant but I would appreciate some general guidance.
The essential bit of code looks like
#define CHUNK 512->4096 depending on implementation
static ssize_t BSL_write(..., const char *buf, size_t count..){ char chunk[CHUNK]; int i,pos, for(i=0,pos=0;i<amount of data;i++,pos+=CHUNK){ copy_from_user(buff,buf+pos,CHUNK); /* reorder data */ /* not significant in throughput */ for(k=0;k<CHUNK;k++){ chunk[k]=buff[B_SM(j+k)]; } memcpy_toio(MEM_reg+pos+i*CHUNK+j,chunk,CHUNK); } return count; } + lots of not important details-----
MEM_reg=ioremap(pci_resource_start(dev,B_SM_MEM)&PCI_BASE_ADDRESS_MEM_MASK,REG_SIZE);
Basically it reads a big buffer from user space, scrambles the byte ordering (according to B_SM) and puts it onto the PCI device memory.
** I currently get 3-4Mb/s throughput (on a fairly poor x86 pc), this is about 1/10 of what I expected to achieve.
** The CHUNK size is not an important factor above 512 bytes- it is contributing to 0.1 percent of the bottle neck. This leads me to think that the copy_from overheads are small.
** The byte ordering is not significant, removing it or replacing it with a null command indicates it contributes ~0.1 percent of the bottleneck.
** The transfer size will always exceed any cache sizes in the system.
I will go onto implementing an mmap method sometime later but as the essential memory / PCI memory bandwidth remains the same I don't think it will make much difference- From testing with different transfer sizes overheads seem small provided a minimum transfer size of 512 bytes is enforced.
Any thoughts? pointers towards profiling this code, typical throughputs, improving throughput etc gratefully received.
Thanks matt
-------- Thanks for the previous help on interrupts etc-
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