Rolling cURL: PHP并发最佳实践

转自：http://www.searchtb.com/2012/06/rolling-curl-best-practices.html

在实际项目或者自己编写小工具(比如新闻聚合,商品价格监控,比价)的过程中, 通常需要从第3方网站或者API接口获取数据, 在需要处理1个URL队列时, 为了提高性能, 可以采用cURL提供的curl_multi_*族函数实现简单的并发.

本文将探讨两种具体的实现方法, 并对不同的方法做简单的性能对比.

1. 经典cURL并发机制及其存在的问题
经典的cURL实现机制在网上很容易找到, 比如参考PHP在线手册的如下实现方式:

function classic_curl($urls, $delay) {
$queue = curl_multi_init();
$map = array();

foreach ($urls as $url) {
// create cURL resources
$ch = curl_init();

// set URL and other appropriate options
curl_setopt($ch, CURLOPT_URL, $url);

curl_setopt($ch, CURLOPT_TIMEOUT, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_NOSIGNAL, true);

// add handle
curl_multi_add_handle($queue, $ch);
$map[$url] = $ch;
}

$active = null;

// execute the handles
do {
$mrc = curl_multi_exec($queue, $active);
} while ($mrc == CURLM_CALL_MULTI_PERFORM);

while ($active > 0 && $mrc == CURLM_OK) {
if (curl_multi_select($queue, 0.5) != -1) {
do {
$mrc = curl_multi_exec($queue, $active);
} while ($mrc == CURLM_CALL_MULTI_PERFORM);
}
}

$responses = array();
foreach ($map as $url=>$ch) {
$responses[$url] = callback(curl_multi_getcontent($ch), $delay);
curl_multi_remove_handle($queue, $ch);
curl_close($ch);
}

curl_multi_close($queue);
return $responses;
}
首先将所有的URL压入并发队列, 然后执行并发过程, 等待所有请求接收完之后进行数据的解析等后续处理. 在实际的处理过程中, 受网络传输的影响, 部分URL的内容会优先于其他URL返回, 但是经典cURL并发必须等待最慢的那个URL返回之后才开始处理, 等待也就意味着CPU的空闲和浪费. 如果URL队列很短, 这种空闲和浪费还处在可接受的范围, 但如果队列很长, 这种等待和浪费将变得不可接受.

2. 改进的Rolling cURL并发方式
仔细分析不难发现经典cURL并发还存在优化的空间, 优化的方式时当某个URL请求完毕之后尽可能快的去处理它, 边处理边等待其他的URL返回, 而不是等待那个最慢的接口返回之后才开始处理等工作, 从而避免CPU的空闲和浪费. 闲话不多说, 下面贴上具体的实现:

function rolling_curl($urls, $delay) {
$queue = curl_multi_init();
$map = array();

foreach ($urls as $url) {
$ch = curl_init();

curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_TIMEOUT, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_NOSIGNAL, true);

curl_multi_add_handle($queue, $ch);
$map[(string) $ch] = $url;
}

$responses = array();
do {
while (($code = curl_multi_exec($queue, $active)) == CURLM_CALL_MULTI_PERFORM) ;

if ($code != CURLM_OK) { break; }

// a request was just completed -- find out which one
while ($done = curl_multi_info_read($queue)) {

// get the info and content returned on the request
$info = curl_getinfo($done['handle']);
$error = curl_error($done['handle']);
$results = callback(curl_multi_getcontent($done['handle']), $delay);
$responses[$map[(string) $done['handle']]] = compact('info', 'error', 'results');

// remove the curl handle that just completed
curl_multi_remove_handle($queue, $done['handle']);
curl_close($done['handle']);
}

// Block for data in / output; error handling is done by curl_multi_exec
if ($active > 0) {
curl_multi_select($queue, 0.5);
}

} while ($active);

curl_multi_close($queue);
return $responses;
}
3. 两种并发实现的性能对比
改进前后的性能对比试验在LINUX主机上进行, 测试时使用的并发队列如下:

http://item.taobao.com/item.htm?id=14392877692
http://item.taobao.com/item.htm?id=16231676302
http://item.taobao.com/item.htm?id=17037160462
http://item.taobao.com/item.htm?id=5522416710
http://item.taobao.com/item.htm?id=16551116403
http://item.taobao.com/item.htm?id=14088310973
简要说明下实验设计的原则和性能测试结果的格式: 为保证结果的可靠, 每组实验重复20次, 在单次实验中, 给定相同的接口URL集合, 分别测量Classic(指经典的并发机制)和Rolling(指改进后的并发机制)两种并发机制的耗时(秒为单位), 耗时短者胜出(Winner), 并计算节省的时间(Excellence, 秒为单位)以及性能提升比例(Excel. %). 为了尽量贴近真实的请求而又保持实验的简单, 在对返回结果的处理上只是做了简单的正则表达式匹配, 而没有进行其他复杂的操作. 另外, 为了确定结果处理回调对性能对比测试结果的影响, 可以使用usleep模拟现实中比较负责的数据处理逻辑(如提取, 分词, 写入文件或数据库等).

性能测试中用到的回调函数为:

function callback($data, $delay) {
preg_match_all('/<h3>(.+)<\/h3>/iU', $data, $matches);
usleep($delay);
return compact('data', 'matches');
}
数据处理回调无延迟时: Rolling Curl略优, 但性能提升效果不明显.

------------------------------------------------------------------------------------------------
Delay: 0 micro seconds, equals to 0 milli seconds
------------------------------------------------------------------------------------------------
Counter Classic Rolling Winner Excellence Excel. %
------------------------------------------------------------------------------------------------
1 0.1193 0.0390 Rolling 0.0803 67.31%
2 0.0556 0.0477 Rolling 0.0079 14.21%
3 0.0461 0.0588 Classic -0.0127 -21.6%
4 0.0464 0.0385 Rolling 0.0079 17.03%
5 0.0534 0.0448 Rolling 0.0086 16.1%
6 0.0540 0.0714 Classic -0.0174 -24.37%
7 0.0386 0.0416 Classic -0.0030 -7.21%
8 0.0357 0.0398 Classic -0.0041 -10.3%
9 0.0437 0.0442 Classic -0.0005 -1.13%
10 0.0319 0.0348 Classic -0.0029 -8.33%
11 0.0529 0.0430 Rolling 0.0099 18.71%
12 0.0503 0.0581 Classic -0.0078 -13.43%
13 0.0344 0.0225 Rolling 0.0119 34.59%
14 0.0397 0.0643 Classic -0.0246 -38.26%
15 0.0368 0.0489 Classic -0.0121 -24.74%
16 0.0502 0.0394 Rolling 0.0108 21.51%
17 0.0592 0.0383 Rolling 0.0209 35.3%
18 0.0302 0.0285 Rolling 0.0017 5.63%
19 0.0248 0.0553 Classic -0.0305 -55.15%
20 0.0137 0.0131 Rolling 0.0006 4.38%
------------------------------------------------------------------------------------------------
Average 0.0458 0.0436 Rolling 0.0022 4.8%
------------------------------------------------------------------------------------------------
Summary: Classic wins 10 times, while Rolling wins 10 times
数据处理回调延迟5毫秒: Rolling Curl完胜, 性能提升40%左右.

------------------------------------------------------------------------------------------------
Delay: 5000 micro seconds, equals to 5 milli seconds
------------------------------------------------------------------------------------------------
Counter Classic Rolling Winner Excellence Excel. %
------------------------------------------------------------------------------------------------
1 0.0658 0.0352 Rolling 0.0306 46.5%
2 0.0728 0.0367 Rolling 0.0361 49.59%
3 0.0732 0.0387 Rolling 0.0345 47.13%
4 0.0783 0.0347 Rolling 0.0436 55.68%
5 0.0658 0.0286 Rolling 0.0372 56.53%
6 0.0687 0.0362 Rolling 0.0325 47.31%
7 0.0787 0.0337 Rolling 0.0450 57.18%
8 0.0676 0.0391 Rolling 0.0285 42.16%
9 0.0668 0.0351 Rolling 0.0317 47.46%
10 0.0603 0.0317 Rolling 0.0286 47.43%
11 0.0714 0.0350 Rolling 0.0364 50.98%
12 0.0627 0.0215 Rolling 0.0412 65.71%
13 0.0617 0.0401 Rolling 0.0216 35.01%
14 0.0721 0.0226 Rolling 0.0495 68.65%
15 0.0701 0.0428 Rolling 0.0273 38.94%
16 0.0674 0.0352 Rolling 0.0322 47.77%
17 0.0452 0.0425 Rolling 0.0027 5.97%
18 0.0596 0.0366 Rolling 0.0230 38.59%
19 0.0679 0.0480 Rolling 0.0199 29.31%
20 0.0657 0.0338 Rolling 0.0319 48.55%
------------------------------------------------------------------------------------------------
Average 0.0671 0.0354 Rolling 0.0317 47.24%
------------------------------------------------------------------------------------------------
Summary: Classic wins 0 times, while Rolling wins 20 times
通过上面的性能对比, 在处理URL队列并发的应用场景中Rolling cURL应该是更加的选择, 并发量非常大(1000+)时, 可以控制并发队列的最大长度, 比如20, 每当1个URL返回并处理完毕之后立即加入1个尚未请求的URL到队列中, 这样写出来的代码会更加健壮, 不至于并发数太大而卡死或崩溃. 详细的实现请参考: http://code.google.com/p/rolling-curl/

5. 参考资料和延伸阅读
Client URL Library http://www.php.net/manual/en/book.curl.php
Parallel CURL Requests with PHP http://blog.rob.cx/multi-curl
A more efficient multi-curl library for PHP (non-blocking) http://code.google.com/p/rolling-curl/
PHP: Parallel cURL Performance http://stackoverflow.com/questions/10485199/php-parallel-curl-performance-rollingcurl-vs-parallelcurl
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