Slow performance in writing to a driver
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- Subject: Slow performance in writing to a driver
- From: Daniel Ng <daniel_ng11@...>
- Date: 2009年5月14日 08:41:09 +0000 (UTC)
Hi,
I'm running lua 5.1.3 on a Linux 2.6 uClibc system.
My Lua script writes to an SPI driver 256 bytes at a time. Unfortunately the
time it takes to get the 256 bytes to the driver is almost 2 seconds!
I need this to write about 2MB of data in total, so this rate is unfortunately
not acceptable.
Please have a look below at a description of the script. Am I doing too much
memory copying? Or could it be something else?
I think memory copying happens when-
-In step 2, when the 2MB is broken into 256 byte chunks
-In step 3, when a new string is effectively created because of the
concatenation
-In step 4, the explicit memcopy()
Cheers,
Daniel
This is what happens in the script:
1) The entire 2MB is read in one go from a file-
local dataIn = file:read("*all")
2) Each 256-byte chunk of the 2MB is then passed to a 'pageProgram()'-
local block = 256
local i = 0
while true do
local bytes = string.sub(dataIn, (i+1), (i+block))
pageProgram(address, dataAddress, bytes)
i = i + block
if i > #dataIn then break end
.
.
.
3) pageProgram() constructs the SPI Message to be passed to the driver:
local spiMessage = write_format(true, PP, dataAddress[1], dataAddress[2],
dataAddress[3]) .. data
-- write_format() has been adapted from:
-- http://lua-users.org/wiki/ReadWriteFormat
4) pageProgram() then calls a C library function to interact with the driver:
int l_spiExchangeBytes(lua_State *L)
{
//The first and second input params deal with basic mangling of the SPI
Address.
.
.
.
//The third input parameter is the Data (represented by a Lua String):
dataIn = (BYTE*) luaL_checkstring(L, 3);
count = lua_objlen(L, 3);
data = (BYTE*) malloc( (sizeof(BYTE)) * count );
//dataIn is a const, so we must memcpy:
if(memcpy(data, dataIn, count) == NULL)
{
...
}
.
.
.
// data is then passed to the SPI driver.
// (note that the memcopy is required because the SPI driver will
// use the same buffer to write back a result, therefore the
// buffer must NOT be a const.)