Diagnosing "ReceiveAckHdr: timeout 300 is exceeded" in mod_lsapi (CloudLinux)

If you run CloudLinux servers with mod_lsapi, you have likely seen this in Apache error_log:

[lsapi:error] [pid 12345:tid 67890] [client 1.2.3.4:0] [host domain.com] Error on sending request(GET / HTTP/1.0); uri(/index.php) content-length(0): ReceiveAckHdr: timeout 300 is exceeded

This error usually means an Apache-PHP bottleneck, not necessarily low CPU or RAM.

What ReceiveAckHdr timeout 300 means

mod_lsapi bridges Apache and LSPHP. Apache forwards the request and waits for an ACK from PHP.

When you see timeout 300, Apache waited 5 minutes and got no useful response from PHP, then terminated the request to protect server capacity.

Step 1: avoid the “resource limit” assumption

Do not raise LVE limits before checking evidence.

• Real resource exhaustion typically shows 508 Resource Limit Reached.
• If lveinfo is low, the issue is likely logical.
• Low CPU with hanging requests points to I/O, DB, or network waits.

Step 2: identify root cause

If PHP stalls for 300 seconds, it is usually waiting on external dependency.

1. Sync plugin loops

In ecommerce stacks, feed plugins may fail to rename temp files and enter loops.

Typical log signal:

PHP Warning: rename(...) No such file or directory

2. Database locks/deadlocks

Blocked queries can freeze multiple PHP workers.

SHOW PROCESSLIST;

or

mysqladmin procstat

3. Slow/unavailable external APIs

Shipping, payment, license, or webhook endpoints can hold PHP until timeout.

Step 3: practical fixes

A) Replace WP-Cron with system cron

Running heavy tasks during web requests increases timeout risk.

In wp-config.php:

define('DISABLE_WP_CRON', true);

Then schedule a real system cron every 5 or 10 minutes.

B) Clean temp and plugin cache artifacts

Review plugin upload/cache folders and remove broken leftovers (.tmp, .csv, .log) when they create permission or I/O conflicts.

C) Align PHP limits

• memory_limit: practical baseline 512M for ecommerce workloads.
• max_execution_time: keep at or below LSAPI timeout to avoid zombie workers.

ReceiveAckHdr: timeout 300 is a stalled backend symptom. Increasing timeout to 600 or 1000 usually delays failure instead of fixing root cause. Real fixes are:

1. plugin loop remediation
2. DB lock/query optimization
3. external API timeout control
4. offloading heavy jobs to system cron

Advanced troubleshooting tip:

strace -p [PHP_PROCESS_PID]

It reveals which syscall is blocking (file, socket, or network), making root cause analysis much faster.

This post is licensed under CC BY-NC.