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Apache showing 100% CPU is a symptom, not a diagnosis. The reliable fix is to identify the active request behind a high-CPU Apache process, determine whether PHP-FPM or WordPress is doing the expensive work, then reduce repeated execution with caching, correct the offending task or plugin, tune concurrency to available memory, and control abusive traffic. Do not begin by blindly raising Apache limits or restarting the server; that can hide the trigger while making memory pressure worse.
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What a 100% Apache CPU reading actually tells you
A process viewer reports how much processor time a process is consuming, but the percentage does not identify the URL, visitor, plugin, or database query responsible. On a multi-core machine, totals can also be interpreted differently by different tools. A PHP-FPM pool may be consuming the processor while Apache appears busy because it is proxying those requests. Conversely, Apache itself may be spending the time serving static files, handling TLS, compressing responses, or waiting on a module.
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Treat the incident as an attribution problem. You need three pieces of evidence taken close together:
- The Apache worker process ID (PID) using CPU.
- The request, client, and connection state attached to that PID.
- Whether the request is waiting on PHP-FPM, WordPress, a database, or Apache-level work.
Requests are transient, so one snapshot can miss the cause. Capture several samples during the spike and compare them with access, error, PHP-FPM, and WordPress timing information.
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Stabilize the site without destroying evidence
When the site is slow but still reachable, collect evidence before restarting services. A restart terminates the request that could have identified the problem. If the machine is in danger of swapping or is not responding, a controlled restart may be necessary, but record the time and preserve logs first if possible.
Immediate checks
- Use
top,htop, or an equivalent process viewer and sort by CPU. Note the Apache parent and worker PIDs, the user account, CPU percentage, memory, and how long each process has been running. - Check memory and swap at the same time. A host that is swapping can make every request appear CPU-bound because the system is spending time recovering from memory pressure.
- Record the time window, current traffic, recent deployments, plugin changes, scheduled jobs, and configuration changes.
- Do not expose diagnostic endpoints or remove authentication as a quick fix.
top -o %CPU
free -h
ps -eo pid,ppid,user,%cpu,%mem,etime,cmd --sort=-%cpu | head -n 25These commands do not identify the URL by themselves. They provide the PIDs you will match to Apache’s status page.
Find the request that owns the hot Apache process
Apache’s mod_status module is the most direct way to connect a worker PID to an active request. The Apache Software Foundation describes server-status as a starting point for situations where the server is consuming all available CPU or memory and you need to identify the requests or clients causing it.
Enable mod_status and protect it
Enable the module using your distribution’s Apache module mechanism, then create a restricted location for /server-status. The endpoint must be available only to an administrator network, a private management address, or localhost. Never leave it open to the public internet: status output can reveal client addresses, request paths, and worker details.
ExtendedStatus On
<Location /server-status>
SetHandler server-status
Require local
# Or replace Require local with a narrowly scoped administrator IP or network.
</Location>The exact authorization directive depends on your Apache version and access-control configuration. Validate the configuration and reload Apache rather than restarting it when the service is healthy.
apachectl configtest
systemctl reload apache2On systems that use a different service name, use the equivalent control command. If ExtendedStatus On is enabled, the status page includes more detailed per-worker information.
Match PIDs to requests
- Run
topand note the highest-CPU Apache PID. - Open the protected
/server-statusendpoint while the spike is occurring. Reload it repeatedly because a request may finish between samples. - Find the matching PID in the scoreboard and record the client address, request line, virtual host, connection state, elapsed time, and bytes transferred.
- Repeat the observation at least several times. A path that appears in multiple samples is more significant than a one-off request.
A worker can change requests between the process-list sample and the status-page view. That is normal; it is why repeated sampling matters. If all workers are short-lived, increase the frequency of observations or use access-log timestamps to reconstruct the burst.
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Interpret the scoreboard carefully
A request that has been running for a long time may be genuinely expensive, blocked on an upstream, or simply transferring a large response. A large number of workers in a waiting state points to an idle-capacity issue rather than a single hot request. A large number actively serving the same uncached URL suggests a request-storm or cache miss pattern. Record both the request path and the client population before deciding that a plugin is at fault.
Separate Apache work from PHP-FPM work
In a common WordPress stack, Apache accepts the connection and passes PHP requests to PHP-FPM. The Apache PID visible in top can therefore be the front end of a slow PHP operation. Diagnose both layers before changing either one.
Use access and error logs to classify the traffic
Search the relevant virtual-host access log for repeated paths, status codes, user agents, response sizes, and bursts in the incident window. Group requests by path and client rather than looking only at total request count. Pay attention to:
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wp-cron.php,admin-ajax.php, REST endpoints, and XML-RPC requests.- Search and filter URLs with many query-string combinations.
- Repeated 404, 403, 500, or redirect responses that indicate a bot loop or broken integration.
- A single user agent or address generating a disproportionate share of requests.
Error logs can reveal upstream timeouts, exhausted worker pools, rewrite loops, permission failures, and module errors. A high rate of 5xx responses is a different remediation path from a high rate of valid, cacheable page views.
Inspect PHP-FPM status and slow requests
Enable PHP-FPM’s status endpoint and slow-request logging only on a protected management path. The status view can show pool saturation, active and idle processes, queueing, and request duration; slow-request logs identify scripts that exceed the configured threshold. Keep the endpoint private because it exposes operational information.
Compare PHP-FPM process IDs and CPU usage with the Apache samples. PHP documentation notes that CPU percentages can exceed 100% when several cores are counted, so interpret the value according to the process viewer’s convention. If PHP-FPM workers are saturated while Apache workers are mostly waiting, changing Apache’s worker count will not fix the expensive PHP path.
When PHP-FPM reports a full pool or a growing queue, investigate the script and its downstream database calls before increasing the pool. More PHP workers can create more simultaneous database work and increase memory pressure.
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Check the WordPress execution paths most likely to overload a server
Uncached anonymous page requests
Without page caching, every anonymous visit can execute Apache, PHP, WordPress bootstrap code, theme logic, and database queries. A burst of legitimate visitors can therefore exhaust CPU even when no single request is broken. Confirm whether the affected responses are cacheable and whether a cache hit is actually being served.
wp-cron.php and loopback requests
WordPress uses loopback requests to start WP-Cron, and plugins or themes can register scheduled events. A site with frequent visits can trigger cron repeatedly; a failed or slow scheduled task can then overlap with new attempts. Look for repeated wp-cron.php entries, long durations, and the same hook or plugin in WordPress’s scheduled-event screen.
For a site that receives steady traffic, administrators often disable visitor-triggered cron and run it from the system scheduler instead. In wp-config.php:
define('DISABLE_WP_CRON', true);Then schedule a command such as the following under the web-server user, adjusting the path and interval to your operating system and workload:
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*/5 * * * * cd /var/www/example && php wp-cron.php >/dev/null 2>&1Do not make this change without confirming that scheduled events are still executed. A system cron that runs too frequently can reproduce the same overload in a more concentrated form.
admin-ajax.php, REST, and XML-RPC
Front-end JavaScript can call admin-ajax.php for every visitor, while editor tools, integrations, and mobile clients may use REST or XML-RPC. These endpoints are usually dynamic and should not be placed behind a generic full-page cache. Identify the action or route in logs, then check which plugin or theme registered it. Rate-limit or restrict an endpoint only after verifying that legitimate features and integrations do not depend on it.
Search, filters, and expensive query strings
On stores, catalogs, and content-heavy sites, combinations of search terms, sorting, and filters can defeat page caching and generate expensive database queries. Compare the query strings associated with long PHP durations and high request volume. Caching the base page will not help if every request has a unique, uncached query string.
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Plugins and themes
A plugin can add scheduled jobs, intercept every request, issue remote API calls, or create unbounded database queries. A theme can do the same through template and hook code. If timing and logs point to WordPress but not to a single route, isolate extensions methodically rather than disabling random components on production.
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- Take a current backup and, if possible, reproduce the issue on staging.
- Record active plugins, their versions, the active theme, and the time of the CPU spike.
- Deactivate all plugins at once in a controlled maintenance window or staging copy. If CPU falls, the extension layer is implicated.
- Reactivate plugins one at a time, allowing enough requests and scheduled events to run after each change.
- When the load returns, inspect that plugin’s hooks, scheduled events, AJAX actions, REST routes, external calls, and database queries.
- Test the same path with the theme changed to a default theme if the plugin sweep is inconclusive.
Do not leave every plugin disabled as the permanent solution. Replace, update, reconfigure, or remove the offending code, and verify that the site’s required functions still work.
Reduce repeated execution with the right cache
Full-page caching stores a rendered response so an anonymous request can be served without executing the complete Apache, PHP, and WordPress stack each time. A reverse-proxy cache can provide the same benefit in front of Apache. This is often the highest-impact remedy when the workload consists of legitimate, repeatable page views.
Choose what may be cached
Cache public GET and HEAD responses that do not vary by user. Exclude paths and cookies that represent private or stateful activity, including:
- Logged-in account pages and administrator screens.
- Cart, checkout, payment, and order-confirmation pages.
- Personalized dashboards, forms, and pages containing user-specific data.
- REST, AJAX, webhook, and other endpoints that perform actions or return per-user data.
After enabling a cache, test anonymous, logged-in, and transactional sessions separately. Verify that cookies, authorization headers, query strings, and purge rules do not accidentally serve one user’s response to another.
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Add persistent object caching when supported
Persistent object caching keeps reusable WordPress query results and objects between requests. It can reduce database work for dynamic pages, but it does not replace full-page caching and does not make an inefficient query inexpensive. Confirm that the host supports the chosen object-cache service, monitor memory usage, and invalidate stale objects after content or configuration changes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Tune Apache concurrency only after identifying the workload
MaxRequestWorkers limits how many requests Apache can process concurrently. Apache documents it as a safeguard against spawning so many children that the system begins swapping. The correct value depends on available RAM, the selected processing model, the memory footprint of each worker, and the cost of the requests.
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Measure before changing the limit
- Record total RAM, swap activity, and the resident memory of representative Apache and PHP-FPM processes.
- Use
mod_statusto see whether workers are busy, idle, or waiting. - Compare the number of simultaneous requests with PHP-FPM’s active and queued workers.
- Check database memory and other services before allocating more to Apache.
A practical ceiling must leave memory for the operating system, PHP-FPM, the database, and filesystem cache. Do not copy a value from another server. If the host swaps, lower concurrency or reduce per-request cost; if workers queue while CPU and memory remain available, a cautious increase may be justified after testing.
Understand the trade-off
Lowering MaxRequestWorkers can keep the machine responsive by limiting simultaneous expensive work, but excess requests will wait or fail and latency can rise. Raising it can improve throughput for cheap requests, yet it can multiply PHP and database work and push the host into swapping. Use status-page observations and controlled load measurements, not CPU percentage alone, to choose the direction.
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If logs show bots, crawlers, repeated scans, or a single client generating needless work, add a rate limit, firewall rule, CDN edge control, or web-application firewall rule appropriate to the traffic. Apply controls as close to the edge as possible so rejected requests do not consume Apache and PHP workers.
Whitelist search engines or partner integrations only when you can verify their identity and need. Challenge or block clients that repeatedly request nonexistent paths, expensive uncached queries, or administrative endpoints. Make sure security rules do not block health checks, payment callbacks, webhooks, or logged-in workflows.
When optimization is not enough: capacity decisions
Sometimes every observed request is legitimate, caching is working, plugins are healthy, and concurrency is already constrained without swapping. That is a capacity problem. Increasing CPU or RAM, moving to a host with more predictable resources, or distributing static and cacheable traffic can be more effective than further Apache changes. Faster SSD or NVMe storage may also reduce I/O wait for databases and logs, but storage speed will not cure a CPU-heavy PHP loop.
Make the capacity decision from measured demand: peak concurrent requests, PHP-FPM queueing, cache-hit ratio, database saturation, memory headroom, and traffic growth. A larger machine should follow workload evidence, not an arbitrary CPU target.
Quick Recap
Remedy comparison
| Remedy | Diagnostic visibility | Cache coverage | Concurrency and memory effect | Traffic protection | Operational control |
|---|---|---|---|---|---|
mod_status plus logs |
Maps Apache PIDs to active requests; PHP-FPM status adds pool and slow-request data. | None; it observes the workload. | No direct change, but prevents tuning by guesswork. | None. | Requires secure Apache and PHP-FPM configuration access. |
| Full-page or reverse-proxy cache | Cache logs and headers show hits and misses; misses still need normal tracing. | Strong for anonymous, cacheable pages; dynamic paths must be excluded. | Reduces repeated Apache, PHP, and WordPress execution. | Can absorb legitimate bursts; it is not a substitute for abuse controls. | Requires correct purge, cookie, header, and exclusion rules. |
| Plugin, theme, or cron isolation | High once a route, hook, or scheduled event is reproduced. | Does not provide cache coverage. | Removes the underlying expensive work rather than merely queuing it. | None. | Needs WordPress administration, staging, and rollback access. |
MaxRequestWorkers adjustment |
Uses status and process memory data; it does not identify the root request alone. | None. | Lower values reduce swapping risk but increase queueing; higher values can increase throughput or exhaust RAM. | None. | Requires Apache configuration access and a reload or restart. |
| Rate limits, CDN, or WAF | Edge logs identify clients and rules; origin logs confirm what reaches Apache. | Can cache or serve static and public responses at the edge. | Rejects or offloads work before it reaches the origin. | Best option for abusive or geographically distributed bursts. | Requires careful exceptions for dynamic and authenticated traffic. |
| More CPU/RAM or a different host | Does not improve attribution. | Depends on the new architecture. | Raises the workload budget but does not remove inefficient code. | May improve headroom; edge controls may still be needed. | Requires migration or infrastructure changes. |
Prevent the next Apache CPU incident
- Keep a protected
mod_statusendpoint and PHP-FPM status view ready before an incident. - Retain access and error logs long enough to compare normal and abnormal periods.
- Monitor Apache active workers, PHP-FPM pool saturation, queue length, response time, cache-hit ratio, memory, swap, and database load.
- Review scheduled WordPress events after installing or updating plugins and themes.
- Test cache exclusions whenever login, checkout, forms, webhooks, or personalization changes.
- Document the Apache MPM,
MaxRequestWorkers, PHP-FPM pool limits, and rollback commands. - Load-test representative anonymous and dynamic URLs after major code or infrastructure changes.
A concise recovery checklist
- Record the CPU, memory, swap, time window, and top Apache and PHP-FPM PIDs.
- Use protected
server-statussampling to associate hot Apache workers with requests and clients. - Group access-log entries by path, status, user agent, query string, and burst pattern.
- Check PHP-FPM status and slow logs to decide whether PHP, WordPress, or Apache is doing the expensive work.
- Inspect uncached pages,
wp-cron.php, loopbacks,admin-ajax.php, REST/XML-RPC, searches, filters, and recent plugin or theme changes. - Isolate a suspected extension on staging or during a controlled window, then fix or remove it.
- Enable correctly scoped page and object caching, with private and transactional paths excluded.
- Set
MaxRequestWorkersfrom measured memory and workload, watching for queueing and swapping. - Rate-limit abusive traffic and add edge protection when the origin is receiving avoidable requests.
- If measured legitimate demand still exceeds the host’s budget, increase capacity or change the architecture.
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