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Linux下异步IO是比较新的内核里面才有的，异步io的好处可以参考这里.
但是文章中aio_*系列的调用是glibc提供的，是glibc用线程+阻塞调用来模拟的，性能很差，千万千万不要用。
我们今天要说的是真正的原生的异步IO接口. 由于这几个系统调用glibc没有提供相应的封装，所以libaio来救急了：
libaio项目: http://oss.oracle.com/projects/libaio-oracle/
This is a library for accessing the new AIO system calls (asynchronous i/o) for the Linux kernel. It is a thin, state-keeping wrapper that conforms to the Single Unix Specification for aio_read, aio_write, aio_error, aio_return and aio_suspend functions, and also implements lio_listio and aio_reap for batch processing.
This library requires a kernel with the new AIO code and a recent version of the libaio userspace library.
libaio提供了4个函数: io_cancal, io_destroy, io_getevents, io_setup,io_submit来提供服务，具体的可以参看man io_*
淘宝的雕梁同学写了这篇文章介绍了libaio如何和event一起高效工作：看这里
使用很简单的,在RHEL 5U4上只要安装下开发包就好:
$ sudo yum install libaio-devel |
其实大家会比较关心性能。 fio 测试工具支持同步(pread/pwrite)和异步(libaio)的测试，那我们比较下:
为了减低底层硬件（如raid）cache的影响，我们用总共2G左右的文件做随机读写操作, 完全随机的读写，块大小128K, directio绕过OS page buffer系统, 总共跑1分钟。
在异步io操作的时候只用一个进程保持io队列的长度为16， 同步io开16个进程，保持同样的队列这样会公平些。
测试机器配置：
# Aspersa System Summary Report ############################## |
Release | Red Hat Enterprise Linux Server release 5.4 (Tikanga) |
Processors | physical = 2, cores = 8, virtual = 16, hyperthreading = yes |
Models | 16xIntel(R) Xeon(R) CPU E5520 @ 2.27GHz |
# Memory ##################################################### |
先开始异步IO的测试：
libaio.dat: (g=0): rw=randrw, bs=128K-128K/128K-128K, ioengine=libaio, iodepth=16 |
Jobs: 1 (f=1): [m] [100.0% done] [31462K/34595K /s] [240 /263 iops] [eta 00m:00s] |
libaio.dat: (groupid=0, jobs=1): err= 0: pid=25892 |
read : io=1795.4MB, bw=30575KB/s, iops=238 , runt= 60129msec |
slat (usec): min=14 , max=130031 , avg=25.13, stdev=1084.85 |
clat (usec): min=178 , max=1959.4K, avg=66231.82, stdev=154044.70 |
lat (msec): min=1 , max=1959 , avg=66.26, stdev=154.04 |
bw (KB/s) : min=22721, max=95552, per=100.66%, avg=30775.78, stdev=7306.36 |
write: io=1836.8MB, bw=31280KB/s, iops=244 , runt= 60129msec |
slat (usec): min=12 , max=55571 , avg=18.50, stdev=458.33 |
clat (usec): min=252 , max=377818 , avg=658.98, stdev=7612.29 |
lat (usec): min=266 , max=377832 , avg=677.73, stdev=7626.25 |
bw (KB/s) : min=20223, max=94275, per=100.74%, avg=31511.06, stdev=7334.59 |
cpu : usr=0.13%, sys=0.72%, ctx=28484, majf=0, minf=24 |
IO depths : 1=0.1%, 2=0.1%, 4=0.1%, 8=0.1%, 16=99.9%, 32=0.0%, >=64=0.0% |
submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0% |
complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.1%, 32=0.0%, 64=0.0%, >=64=0.0% |
issued r/w/d: total=14363/14694/0, short=0/0/0 |
lat (usec): 250=0.01%, 500=46.44%, 750=1.91%, 1000=0.39% |
lat (msec): 2=1.55%, 4=0.39%, 10=3.07%, 20=5.66%, 50=20.97% |
lat (msec): 100=14.42%, 250=4.46%, 500=0.19%, 750=0.02%, 1000=0.01% |
Run status group 0 (all jobs): |
READ: io=1795.4MB, aggrb=30575KB/s, minb=31309KB/s, maxb=31309KB/s, mint=60129msec, maxt=60129msec |
WRITE: io=1836.8MB, aggrb=31279KB/s, minb=32030KB/s, maxb=32030KB/s, mint=60129msec, maxt=60129msec |
sda: ios=15108/20465, merge=4704/4206, ticks=969251/16763, in_queue=989615, util=99.84% |
26112 io_submit(47710156091392, 1, {{(nil), 0, 1, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 1, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 1, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 1, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 1, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 1, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 1, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_submit(47710156091392, 1, {{(nil), 0, 0, 0, 9}}) = 1 |
26112 io_getevents(47710156091392, 1, 1, {{(nil), 0x18ad9f00, 131072, 0}}, NULL) = 1 |
再看下同步IO的测试：
file1: (g=0): rw=randrw, bs=128K-128K/128K-128K, ioengine=psync, iodepth=1 |
file1: (g=0): rw=randrw, bs=128K-128K/128K-128K, ioengine=psync, iodepth=1 |
Jobs: 16 (f=16): [mmmmmmmmmmmmmmmm] [100.0% done] [31854K/31723K /s] [243 /242 iops] [eta 00m:00s] |
file1: (groupid=0, jobs=1): err= 0: pid=26145 |
read : io=118144KB, bw=1967.1KB/s, iops=15 , runt= 60036msec |
clat (msec): min=3 , max=1531 , avg=64.36, stdev=133.07 |
lat (msec): min=3 , max=1531 , avg=64.36, stdev=133.07 |
bw (KB/s) : min= 84, max= 6083, per=7.02%, avg=2205.14, stdev=807.87 |
write: io=121984KB, bw=2031.9KB/s, iops=15 , runt= 60036msec |
clat (usec): min=264 , max=153817 , avg=660.30, stdev=6100.86 |
lat (usec): min=265 , max=153818 , avg=660.53, stdev=6100.88 |
bw (KB/s) : min= 84, max= 5765, per=7.32%, avg=2311.53, stdev=1256.97 |
cpu : usr=0.00%, sys=0.05%, ctx=1877, majf=0, minf=32 |
IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0% |
submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0% |
complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0% |
issued r/w/d: total=923/953/0, short=0/0/0 |
lat (usec): 500=46.75%, 750=2.40%, 1000=0.43% |
lat (msec): 2=1.01%, 4=0.16%, 10=2.99%, 20=4.69%, 50=20.31% |
lat (msec): 100=15.41%, 250=5.28%, 500=0.21%, 2000=0.37% |
Run status group 0 (all jobs): |
READ: io=1844.0MB, aggrb=31411KB/s, minb=1699KB/s, maxb=2186KB/s, mint=60001msec, maxt=60114msec |
WRITE: io=1852.9MB, aggrb=31562KB/s, minb=1620KB/s, maxb=2325KB/s, mint=60001msec, maxt=60114msec |
sda: ios=15756/20708, merge=4553/4159, ticks=979074/16483, in_queue=999501, util=99.83% |
26195 pwrite(20, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 131072, 5242880) = 131072 |
26195 pwrite(20, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 131072, 23855104) = 131072 |
26195 pwrite(20, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 131072, 7864320) = 131072 |
26195 pread(20, <unfinished ...> |
从上面的数据我们可以看出，最后的结果都是设备的IO能力将近100%的被使用，二个测试的吞吐量差不多。 但是如果从当个进程提交的吞吐量来看的话，一个aio进程相当于16个同步进程的能力，也就是说如果大家都是一个进程在干活的话，那么aio的能力要比同步的好太多了。
大家玩得开心！
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