select实现高精度定时器
2011-08-04 16:45阅读：
select实现高精度定时器
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因为各种各样的原因，有时候必须把超时和socket的事件放在一个线程里，也就是说不可避免的要用select做超时功能，
经过测试，select在windows下的timeout功能精度太低，大概在15ms左右，在linux下精度在1ms，我记得几年前就是这种情况，几年过去了，MS还是这个德性，一点长进都没有。
select函数本身限制够多的，比如可查询的handle数，linux下不重新编译内核的话，是1024，但是linux下有没有这个限制，而且性能更高的函数可选择。windows根本就没辙，就这样了，自己想点其他办法。
因为这个原因，几年前毅然选择了linux。
但是现在没辙了，大环境在windows，只能想点歪门邪道了。
在windows下，经测试select超时精度在15ms，timeval.tv_usec设为1，1000，10000，都一样，14ms多超时，说明精度在15ms，但是为0的话，一般10us内超时。
但是我的要求是select的精度在1ms。得想办法来。
select的句柄有事件的话，select的返回挺快的，这是不是有可利用的地方，答案就是这个。
create两个dummy
handle与windows的1ms精度的多媒体定时器配合使用，使select超时达到1ms的精度。
zhaohongxian@gmail.com
伪代码如下：
1. create ReadDummySocket
2. bind
ReadDummySocket
3. create WriteDummySocket
4. call timeGetDevCaps() get windows multimedia timer
resolution
5.
steps in select timeout funct
ion
5.1 calculate timeout for select() function;
5.2 call timeSetEvent() function to set a windows multimedia
timer.
5.3 add ReadDummySocket to readset fdset, and call select() to
query handle's events or timeout
5.4 send a dummy packet to ReadDummySocket through WriteDummySocket
in the CALLBACK functin of windows multimedia timer
5.5 select return when ReadDummySocket has a readable event
5.6 call timeKillEvent() to kill the windows multimedia timer id
return in step 5.2
parts of .h file
int dummyReadSocket;
int dummyWriteSocket;
unsigned uTimeResolution;
unsigned nTimeId;
int readPort;
parts of .cpp file
extern 'C' void CALLBACK TestTimeProc(UINT id, UINT msg, DWORD
dwUser, DWORD dw1, DWORD dw2 )
{
NotifyDummyReadSocket();
}
void InitMMTimer()
{
dummyReadSocket = socket(AF_INET, SOCK_DGRAM, 0);
dummyWriteSocket = socket(AF_INET, SOCK_DGRAM,
0);
struct sockaddr_in readAddr;
readAddr.sin_family = AF_INET;
readAddr.sin_addr.s_addr = inet_addr('127.0.0.1');
readPort = -1;
for( int i=31313; i<35000;
i++)
{
readAddr.sin_port = htons(i);
if( 0 == bind(dummyReadSocket,
(struct sockaddr*)&readAddr, sizeof(readAddr)) )
{
readPort = i;
break;
}
}
FD_SET((unsigned)dummyReadSocket,
&fReadSet);
TIMECAPS tc;
if( TIMERR_NOERROR == timeGetDevCaps(&tc, sizeof(tc))
)
{
uTimeResolution = min(max(tc.wPeriodMin,1),
tc.wPeriodMax);
timeBeginPeriod(uTimeResolution);
}
}
void SetMMTimer(UINT microSecond)
{
UINT milliSeccond = microSecond / 1000;
if( milliSeccond == 0 )
{
return;
}
if( milliSeccond < uTimeResolution
)
{
milliSeccond = uTimeResolution;
}
if( milliSeccond > 320 )
{
milliSeccond = 320;
}
nTimeId = timeSetEvent(milliSeccond,
uTimeResolution, TestTimeProc, (DWORD_PTR)this,
TIME_ONESHOT);
}
void KillMMTimer()
{
if( nTimeId != NULL )
{
timeKillEvent(nTimeId);
}
}
void NotifyDummyReadSocket()
{
if( readPort == -1 )
{
return;
}
struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr('127.0.0.1');
addr.sin_port = htons(readPort);
sendto(dummyWriteSocket, '1', 1, 0,
(struct sockaddr*)&addr, sizeof(addr));
}
void SelectThreadProc()
{
...
unsigned uTimeout = tv_timeToDelay.tv_sec * 1000000 +
tv_timeToDelay.tv_usec;
SetMMTimer(uTimeout);
//int nDiff = 0;
//gettimeofday(&start, NULL);
int selectResult =
select(fMaxNumSockets, &readSet, &writeSet,
&exceptionSet, &tv_timeToDelay);
...
KillMMTimer();
...
if( FD_ISSET(dummyReadSocket, &readSet) )
{
char buf[128];
recvfrom(dummyReadSocket, buf, 128, 0, NULL,
NULL);
}
}
