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服務(wù)器負荷增大，嘗試優(yōu)化和調整服務(wù)器，首先是從 Apache 的優(yōu)化和調整開(kāi)始的。之前考慮到我們的生產(chǎn)機負荷并不大，因此可以讓apache的每一個(gè)派生子進(jìn)程都盡可能處理更多的請求再釋放，這樣有利于促進(jìn)子進(jìn)程的復用，從而提高處理效率。于是我將apache的MaxRequestsPerChild參數調整為4000，一直大半年相安無(wú)事。
但是今天卻出現了一個(gè)問(wèn)題，網(wǎng)站突然打不開(kāi)了，癥狀是瀏覽器始終處于正在打開(kāi)網(wǎng)頁(yè)的狀態(tài)。檢查后排除了DDoS攻擊的可能。再查看apache進(jìn)程，居然沒(méi)有！趕緊查看apache錯誤日志/var/log/httpd/error_log，發(fā)現了以下錯誤日志：

[Tue May 08 12:21:22 2012] [alert] (11)Resource temporarily unavailable: setuid: unable to change to uid: 48
[Tue May 08 12:21:23 2012] [alert] Child 8762 returned a Fatal error... Apache is exiting!
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (43)Identifier removed: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (22)Invalid argument: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (22)Invalid argument: couldn't grab the accept mutex
[Tue May 08 12:21:24 2012] [emerg] (22)Invalid argument: couldn't grab the accept mutex
[Tue May 08 12:21:25 2012] [emerg] (22)Invalid argument: couldn't release the accept mutex
[Tue May 08 15:39:47 2012] [emerg] (22)Invalid argument: couldn't grab the accept mutex
[Tue May 08 15:39:47 2012] [emerg] (22)Invalid argument: couldn't grab the accept mutex
[Tue May 08 15:39:47 2012] [emerg] (22)Invalid argument: couldn't grab the accept mutex
[Tue May 08 15:39:47 2012] [emerg] (22)Invalid argument: couldn't grab the accept mutex
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從時(shí)間上看，正是這次故障發(fā)生的時(shí)間點(diǎn)。重新啟動(dòng)apache后恢復正常。最后發(fā)現是 MaxRequestsPerChild 參數設置不合理。再次仔細閱讀 Apache 手冊關(guān)于 MaxRequestsPerChild 的說(shuō)明后，認為在 prefork 下，KeepAlive 開(kāi)啟的情況下，每一個(gè)新連接都會(huì )導致一個(gè) Apache 子進(jìn)程開(kāi)啟， MaxRequestsPerChild 設置過(guò)大，導致新連接產(chǎn)生新的子進(jìn)程后，長(cháng)期空閑 ，這樣并不一定合理。

根據我們網(wǎng)站的訪(fǎng)問(wèn)情況，其實(shí)遠遠不需要那么大的 MaxRequestsPerChild 值，每個(gè)用戶(hù)點(diǎn)擊網(wǎng)站的頻率也不會(huì )很高，所以 MaxRequestsPerChild 設置得太高了反而適得其反，相反應該盡快釋放連接，盡快回收系統資源，以盡可能快速的滿(mǎn)足新的請求的連接需要。于是將 MaxRequestsPerChild 重新調整為64，估計不會(huì )再有這個(gè)問(wèn)題產(chǎn)生了，具體情況是否如此還需要長(cháng)期的觀(guān)察。