我是用SATA磁盘对Linux机箱上的同步读取性能进行基准测试。使用单线程读取,奇怪的是更高的QPS(50)在读取300个条目后给出了12ms的平均读取时间,而在读取相同的300个条目之后较低的QPS(1)给出了63ms。有没有解释?为什么更频繁地读取磁盘会使Linux上的每个读取操作更快? QPS1与50
代码和数据如下:
struct Request{
unsigned long long len;
unsigned long long offset;
int fd;
};
int read_request(Request* request){
char* buf = (char*)malloc(request->len);
off_t of = lseek(request->fd,request->offset,SEEK_SET);
assert(of == request->offset);
int len = read(request->fd,buf,request->len);
assert(len == request->len);
free(buf);
return 0;
}
int read_with_qps(Request* request,int request_num,Files* f,int mode,int qps){
int interval = 1000/qps;
struct timeval start,end;
for(int i = 0 ; i < request_num ; i++){
gettimeofday(&start,NULL);
int ret = read_request(&request[i]);
gettimeofday(&end,NULL);
int time_used = (end.tv_sec - start.tv_sec) * 1000 + (end.tv_usec - start.tv_usec)/1000;
fprintf(stderr,"%lld,offset=%lld,len=%lld, read time:%d,ret=%d,mode=%d\n",
end.tv_sec,request[i].offset,request[i].len,time_used,ret,mode);
if(time_used < interval){
usleep((interval - time_used) * 1000);
}
}
return 0;
}
随着QPS = 50时,输出的采样的样子(忽略时间,这被认为是计算平均时间时,击中页面缓存< 4ms的):
1332233329,offset=1052299215,len=13186, read time:13,ret=0,mode=1
1332233329,offset=2319646140,len=1612, read time:10,ret=0,mode=1
1332233330,offset=1319250005,len=5654, read time:12,ret=0,mode=1
1332233330,offset=2520376009,len=2676, read time:12,ret=0,mode=1
1332233330,offset=2197548522,len=17236, read time:10,ret=0,mode=1
1332233330,offset=1363242083,len=13734, read time:11,ret=0,mode=1
1332233330,offset=4242210521,len=2003, read time:17,ret=0,mode=1
1332233330,offset=1666337117,len=2207, read time:10,ret=0,mode=1
1332233330,offset=797722662,len=5480, read time:18,ret=0,mode=1
1332233330,offset=1129310678,len=2265, read time:10,ret=0,mode=1
QPS = 1,smaple的相同提取物:
1332300410,offset=1052299215,len=13186, read time:19,ret=0,mode=1
1332300411,offset=2319646140,len=1612, read time:40,ret=0,mode=1
1332300412,offset=1319250005,len=5654, read time:141,ret=0,mode=1
1332300413,offset=2520376009,len=2676, read time:15,ret=0,mode=1
1332300414,offset=2197548522,len=17236, read time:21,ret=0,mode=1
1332300415,offset=1363242083,len=13734, read time:13,ret=0,mode=1
1332300416,offset=4242210521,len=2003, read time:43,ret=0,mode=1
1332300417,offset=1666337117,len=2207, read time:18,ret=0,mode=1
1332300418,offset=797722662,len=5480, read time:67,ret=0,mode=1
1332300419,offset=1129310678,len=2265, read time:12,ret=0,mode=1
内核版本我S:2.6.18-194.el5 SMP x86_64的
$ cat /sys/block/sda/queue/scheduler
noop anticipatory deadline [cfq]
感谢您的回复
但是读取请求是同步的,即下一次读取在最后一次读取返回之前不会发出。 – Utoah 2012-03-21 03:55:25
对于偏移量,请求的高度随机性如示例输出所示。 – Utoah 2012-03-21 03:58:05