SDIO small buffer write speed

Question

Posted on June 20, 2015 at 09:10Good day,I have a bit of a problem, is it possible to use SDIO with smaller buffers and have decent speed.I tried to do few tests with smaller buffer like data_buffer[512], data_buffer[1024]...And have really poor speed. My SDIO clock if I remember correctly is 24MHz.512B   buffer speed: 84.545532   KB/s, done 8388608 bytes1024B  buffer speed: 242.144394  KB/s, done 8388608 bytes2048B  buffer speed: 641.919800  KB/s, done 8388608 bytes4096B  buffer speed: 1517.201660 KB/s, done 8388608 bytes8192B  buffer speed: 3362.167480 KB/s, done 8388608 bytes16384B buffer speed: 5949.367188 KB/s, done 8388608 bytes32768B buffer speed: 7989.150391 KB/s, done 8388608 bytesThank you,//Nikolaj

Tesla DeLorean · Answer

Posted on June 20, 2015 at 16:40There tends to be a lot of command/protocol overhead, vs streaming data. The SD cards has something like an 8051 processing commands, and care-taking the NAND array, managing blocks, erase, etc. It's not very rapid, and the erase block size is in the order of 128KB. The streaming of the data into internal buffers is quite rapid, in the order of 12 MBps. You basically need to move away from doing small blocks, either as you buffer the data you want going out, or creating some sort of lazy writer or cache to consolidate blocks going to the media. If I was generating logs files, I would tend to avoid fwrite(), fprintf() type constructs, recording small buffers, into a larger buffer, and once that got sufficiently full flush that to the media. Basically trading the cost of memory-to-memory copies, against the penalty of doing multiple small writes. Obviously the reduce memory in embedded systems tends to skew things also.

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