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Fix Apache crashes and MaxRequestWorkers alerts by treating them as two related but distinct problems: first capture the failure, identify the active MPM, and check memory and upstream latency; then tune concurrency only to the level the host can sustain. A message that Apache has reached MaxRequestWorkers means no worker process or thread was immediately available for another request. It does not, by itself, prove that increasing the limit is safe or appropriate.
What the MaxRequestWorkers message actually means
Apache logs a message such as server reached MaxRequestWorkers setting, consider raising MaxRequestWorkers when all request-serving capacity allowed by the active multi-processing module (MPM) is occupied. New requests may wait in a queue, time out, or be rejected depending on the rest of the configuration and the workload.
The Apache Software Foundation defines MaxRequestWorkers as the maximum number of simultaneous requests that will be served. In the prefork MPM, the number corresponds to child processes. In the worker and event MPMs, it is the total number of request-serving threads. The directive is therefore a concurrency ceiling, not a performance target.
- A low ceiling can make otherwise healthy requests wait during a burst.
- A high ceiling can let too many processes or threads compete for RAM and CPU.
- Slow database, PHP, proxy, or network operations can occupy workers long enough to trigger the message even when request arrival is not unusually high.
- A crash can occur at the same time as the message without the limit being the cause. A child process exiting with a segmentation fault, for example, requires a separate investigation.
Apache’s current 2.5 module documentation (Apache Software Foundation, 2026) documents a prefork default of 256. For worker and event, it documents a relationship of 16 ServerLimit times 25 ThreadsPerChild, producing 400 workers. Those are Apache documentation defaults, not promises about an Ubuntu release or your installed build; inspect the active configuration before drawing conclusions.
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Use this triage sequence before changing a limit
- Capture the event. Record the time, URL or application operation affected, HTTP status, and whether the symptom was a crash, a timeout, or only a warning.
- Check service state and the boot journal. Run the commands in the next section while the evidence is still available.
- Read Apache’s error log. It is normally
/var/log/apache2/error.log, unless the configuredErrorLogpoints elsewhere. - Identify the loaded MPM and modules. A limit has a different meaning in
preforkthan inworkerorevent. - Check host pressure. Compare memory, swap, CPU, process or thread counts, request latency, and upstream wait time at the incident timestamp.
- Validate any proposed change. Run a configuration test, reload when possible, and keep the previous file available for rollback.
Do not begin by copying a larger value from another server. Two machines with the same traffic can require very different limits if one loads a memory-heavy PHP integration, proxy module, or monitoring agent.
Capture startup, runtime, and kernel evidence
The Apache documentation says the server error log is the first place to look when a problem occurs while starting or running the server. On Ubuntu, collect that log together with systemd and kernel records so you can correlate one failure across all layers.
sudo systemctl status apache2 --no-pager
sudo journalctl -u apache2 -b --no-pager
sudo tail -n 200 /var/log/apache2/error.logIf ErrorLog is customized, obtain its path from the active virtual-host and server configuration rather than assuming the default file. Preserve timestamps; a child-process exit in the Apache log should be compared with the same minute in the service and kernel journals.
Search the relevant window for signatures that distinguish exhaustion from a code or module fault:
sudo grep -E -i 'AH00161|MaxRequestWorkers|segfault|child pid|out of memory|OOM|signal|proxy_fcgi' /var/log/apache2/error.logAlso inspect kernel messages for the Linux out-of-memory killer and other process failures:
sudo journalctl -k -b --no-pagerLook for terms such as Out of memory, oom-kill, a killed Apache or PHP process, or a kernel-reported segmentation fault. If the machine uses a separate log destination, use that destination instead. A clean Apache log with an OOM event in the kernel journal points toward host-wide memory pressure; a repeated Apache segfault with no memory pressure points more strongly toward a module, library, or integration defect.
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Identify the active MPM before interpreting any setting
Unix Apache builds use one of three MPMs: prefork, worker, or event. Exactly one MPM may be loaded. Ubuntu installations often use event on current releases, but the enabled module and compiled defaults on your host are authoritative.
apache2ctl -M
apache2ctl -VIn the module list, look for one of mpm_prefork_module, mpm_worker_module, or mpm_event_module. On Ubuntu, inspect /etc/apache2/mods-enabled/ and its MPM symlink to see which configuration file is enabled. Loading two MPMs produces a startup configuration error, so do not enable a second one as an experiment.
| MPM | What the worker count represents | Important tuning constraint | Operational concern |
|---|---|---|---|
prefork |
One simultaneous request per child process | Values above 256 need a corresponding ServerLimit ceiling |
Each child is a process and can consume substantial private memory; process growth can exhaust RAM quickly |
worker |
Total simultaneous request-serving threads across child processes | MaxRequestWorkers should align with ThreadsPerChild; invalid values are rounded down |
Account for per-process memory, thread stacks, and module behavior |
event |
Total request-serving threads, with connection handling optimized for keep-alive traffic | Use the same ThreadsPerChild relationship and multiple-of-that value rule |
Thread safety and the behavior of loaded modules still determine whether the model is suitable |
Changing from prefork to a threaded MPM is not a generic crash fix. Check PHP and every loaded or integrated module for thread-safety, document the old symlinks and settings, and plan a rollback before making the switch. Make the change in a maintenance window if the current service is unstable.
Validate the configuration without making an outage worse
Before a reload or restart, test syntax and module availability:
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sudo apache2ctl configtestA successful result should report Syntax OK. Fix duplicate directives, missing include files, invalid module references, and MPM conflicts before applying a limit change. Keep a copy of the last known-good configuration or use version control so you can restore it quickly.
When the process model does not need to be replaced, apply a valid change with a graceful reload:
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sudo systemctl reload apache2Use a full restart only when the module or process-model change requires it, or when a controlled restart is part of recovering stuck processes. Immediately check service state and the error log after either operation:
sudo systemctl status apache2 --no-pager
sudo journalctl -u apache2 -n 100 --no-pagerIf the test fails, do not repeatedly restart Apache. Restore the previous configuration, run configtest again, and only then attempt another reload.
Decide whether the ceiling is the symptom or the cause
A repeated limit message is actionable, but the correct action depends on what occupies the workers. Measure during a representative busy period rather than relying on a single quiet inspection.
- Request latency: Long-running requests consume slots longer. Break out application, database, file, and upstream wait time if your access logs or tracing system provides it.
- Keep-alive behavior: Connections that remain open can affect available capacity, particularly when clients or intermediaries hold connections while sending data slowly.
- Upstream waits:
proxy_fcgi, PHP-FPM, database calls, and other backends can leave Apache workers occupied while the actual bottleneck is elsewhere. - Memory and swap: Watch resident memory, swap use, major paging, and the point at which latency rises. A larger limit that pushes the host into swap is not a successful fix.
- CPU and run queue: If CPU is already saturated, adding concurrency usually increases contention rather than throughput.
- Process and thread counts: Compare the observed number with the MPM’s configured process and thread ceilings.
- HTTP errors: Track 5xx responses and upstream failure rates before and after each change.
| Evidence at the incident time | More likely interpretation | First response |
|---|---|---|
| Workers are full, RAM and swap have headroom, and requests are short | The ceiling may be limiting burst concurrency | Increase in a small, measured step after calculating a safe memory budget |
| Workers are full while upstream requests are slow | Apache is waiting on the application or proxy backend | Investigate backend capacity, slow queries, timeouts, and connection pools before raising the ceiling |
| Workers are full and the host is swapping or reports OOM kills | The configured concurrency is beyond host capacity | Reduce concurrency or memory per request and fix the memory-hungry component |
Child processes exit with segfault or a signal |
A crash or binary/module fault may coexist with capacity exhaustion | Correlate kernel and core-dump evidence and isolate the changed or implicated module |
| Apache fails immediately after editing a limit | Syntax, MPM, or ServerLimit relationship is invalid |
Restore the prior file, run configtest, and correct the MPM-specific constraint |
Estimate a safe limit from observed memory
For prefork, estimate the resident memory of an Apache child after the real modules and application integration are loaded. Reserve memory for the operating system, databases, PHP-FPM, proxy backends, logging, monitoring, and short-lived bursts. A practical starting calculation is:
safe children ≈ memory reserved for Apache children ÷ observed resident memory per childUse measurements from the affected host and workload, not a generic blog formula. Shared pages can make simple process-size arithmetic misleading, while copy-on-write changes the footprint as children handle requests. Recheck resident memory after traffic has warmed caches and exercised the largest responses.
For worker and event, account for both the memory of each child process and thread-stack and module behavior. The process count may be much lower than the request count, but the total number of active threads still determines concurrency. Leave headroom for the kernel and for non-Apache services; a setting that fits only under an idle snapshot is unsafe.
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Increase one variable at a time, record the old and new values, and observe memory, swap, latency, and 5xx responses through a representative traffic period. If the host begins swapping or the kernel reports OOM activity, lower concurrency and address the component consuming memory instead of continuing to raise MaxRequestWorkers.
Apply MPM-aware settings
When the active MPM is prefork
In prefork, MaxRequestWorkers is the maximum number of child processes that can serve requests at once. The Apache 2.5 documentation lists 256 as its default. That value is not an Ubuntu guarantee and may be overridden by the enabled configuration.
Values above 256 require a suitable ServerLimit. Treat both directives as a pair, and raise ServerLimit only as far as the measured memory budget requires. Apache warns that unnecessarily high limits allocate additional shared memory and that values beyond system capacity can prevent startup or destabilize the host.
# Example structure; choose values from measurements, not from this snippet
<IfModule mpm_prefork_module>
ServerLimit 300
MaxRequestWorkers 300
</IfModule>The numbers in this example illustrate the relationship only; they are not a recommendation for your server. If your measured safe process count is below the current setting, lowering the ceiling may be the correct repair.
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In the threaded MPMs, MaxRequestWorkers is the total request-serving thread ceiling. It should be at least ThreadsPerChild and normally an integer multiple of it. Apache rounds an invalid value down and logs a warning, so a value that looks close to your target may silently provide less capacity than expected.
# Keep MaxRequestWorkers aligned with ThreadsPerChild
<IfModule mpm_event_module>
ThreadsPerChild 25
MaxRequestWorkers 400
</IfModule>Again, the values shown mirror the documented relationship and are not an Ubuntu tuning prescription. Select a process count, thread count, and total concurrency that fit measured memory and CPU headroom. Check the active file before editing because distribution defaults and local overrides vary.
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Do not use ServerLimit as a blanket cure
ServerLimit is a hard ceiling on the number of server processes that the MPM can create. Raising it does not make requests faster and does not solve slow upstream calls. Apache explicitly warns that setting both ServerLimit and MaxRequestWorkers above what the system can handle may keep Apache from starting or make the system unstable.
Investigate crashes independently from capacity alerts
Search for segfault, signal, repeated child-PID exits, abrupt restarts, and module names in the Apache and kernel logs. A limit message can be the last visible event before a crash simply because the server was busy; that timing does not establish causation.
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- Compare
journalctl -koutput with Apache’s error log for the same timestamp. - Check core-dump records if the host is configured to retain them, for example with
coredumpctl list. - Compare the loaded module list with recent package, PHP, TLS, proxy, protocol, or configuration changes.
- In a controlled maintenance window, isolate a recently changed third-party module or integration and retest with a reversible configuration.
Do not disable security or protocol features broadly on a production server just to make a crash disappear. Change one component at a time, preserve the rollback state, and distinguish a module fault from memory exhaustion by checking whether the kernel killed the process and whether resident memory was already at the host limit.
Check upstream services and connection behavior
Apache can reach its concurrency ceiling while the real bottleneck is an application server or backend. For configurations using proxy_fcgi, inspect PHP-FPM pool limits, slow requests, backend connection availability, and application logs at the same timestamps. For other proxies, check backend health, queue depth, timeout events, and connection pools.
Review keep-alive and timeout behavior when many workers appear occupied by idle or very slow clients. The goal is not to turn every timeout down blindly: an aggressive timeout can terminate legitimate long requests, while a generous one can let stalled work consume scarce slots. Choose values from observed request and upstream behavior, then verify that the change reduces occupied-worker time without increasing 5xx responses.
If backend saturation is confirmed, increasing Apache’s ceiling can amplify the surge sent to that backend. Remediate the slow service, query, pool, or network path first, or cap concurrency at the layer that protects the constrained resource.
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| Strategy | Use it when | Risks and checks | Rollback |
|---|---|---|---|
Increase MaxRequestWorkers |
Workers are exhausted, requests are healthy, and measured RAM and CPU headroom remains | Can trigger swapping, OOM kills, longer queues, or backend overload; confirm MPM-specific rules | Restore the previous directive and reload after configtest |
| Change MPM | The current process model is incompatible with the workload or module set and a tested alternative is available | Requires module thread-safety checks, configuration changes, and a maintenance window; exactly one MPM may be loaded | Re-enable the documented prior MPM symlink and configuration |
| Remediate upstream or request behavior | Workers are waiting on PHP, databases, proxies, slow clients, or long application operations | Requires application or backend evidence; changing Apache alone can hide the bottleneck | Revert the backend or timeout change independently and compare latency and error rate |
Make the comparison using the same evidence for every option: MPM compatibility, memory per process or thread, peak concurrency and queueing, keep-alive and upstream wait time, module thread-safety, crash signatures, rollback complexity, measured latency, and 5xx rate.
Verify the repair under representative traffic
- Run
sudo apache2ctl configtestand require a successful result. - Reload Apache when possible, then confirm
sudo systemctl status apache2 --no-pagerreports a healthy service. - Watch
/var/log/apache2/error.logand the systemd journal for repeatedMaxRequestWorkers, child exits, startup warnings, or module errors. - Observe RAM, swap, CPU, process and thread counts, request latency, upstream health, and 5xx responses through a representative busy period.
- Compare the measurements with the pre-change window. A lower warning count is not enough if latency, swapping, or errors increased.
- Document the active MPM, each limit, the evidence used to select it, the observation window, and the exact rollback command.
If the warning returns only during a known burst and the host remains healthy, continue capacity planning rather than making an unmeasured permanent increase. If crashes continue, revert the limit change and focus on the correlated signal, module, or upstream service.
Quick Recap
Prevent the next incident
- Keep the active MPM and limit settings in version control with the Ubuntu package and Apache version noted.
- Alert on sustained worker saturation, swap growth, OOM events, rising latency, and 5xx responses together rather than on the Apache warning alone.
- Record whether a limit is a documented default, a local override, or a value selected from measured memory data.
- Review module and PHP changes as possible crash triggers, especially after a package upgrade or process-model change.
- Test reload and rollback procedures before a high-traffic event.
- Recalculate capacity after adding a database, proxy, monitoring agent, TLS feature, or memory-intensive application component.
Commands reference
| Purpose | Command |
|---|---|
| Service state | sudo systemctl status apache2 --no-pager |
| Apache journal for the current boot | sudo journalctl -u apache2 -b --no-pager |
| Loaded modules and MPM | apache2ctl -M |
| Build and compile-time details | apache2ctl -V |
| Configuration validation | sudo apache2ctl configtest |
| Graceful application of a valid change | sudo systemctl reload apache2 |
| Kernel and OOM evidence | sudo journalctl -k -b --no-pager |
| Core-dump inventory, when enabled | coredumpctl list |
