To find the biggest memory users quickly, run ps -eo pid,comm,rss,%mem --sort=-rss | head -20. It lists processes from highest resident memory (RSS) to lowest. For a live view, use top or htop and sort the process list by memory. Treat RSS as a useful first comparison, not an exact measure of each process’s exclusive share of RAM: shared pages can appear in multiple processes’ totals.
Get a quick ranking of memory-heavy processes
Use a one-time snapshot
On systems with procps-style ps, run:
ps -eo pid,comm,rss,%mem --sort=-rss | head -20The columns show the process ID (PID), command name, resident set size in kilobytes (RSS), and the process utility’s memory percentage. The list is sorted by RSS, largest first. This is a snapshot: a process may move up or down as its workload changes. If your version of ps does not accept these options or displays different columns, check its local manual page.
Watch a changing workload
Run top for a live process view. If htop is installed, it provides an interactive alternative. Sort the process list by a memory field; the available fields and controls vary by utility version and configuration. Consult the top manual or the htop manual if you need the exact controls for your installation.
Choose the right memory figure
Process tools expose several memory measurements. They answer different questions, so the largest number is not always the most useful one.
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| Field | What it tells you | How to use it |
|---|---|---|
| RSS or RES | Resident pages currently in memory for a process. Shared pages may be counted in the RSS of more than one process. | Use it for a quick ranking, but do not add process RSS figures and treat the sum as exact system RAM usage. Linux kernel documentation describes the accounting limitations. |
| PSS | Proportional set size: a process’s unshared memory plus its proportional share of shared pages. | Use it when shared-page attribution matters and you want less duplication across process totals. The smem manual describes the measure. |
| USS | Unique set size: memory not shared with other processes. | Use it to examine the memory exclusive to a process. PSS and USS describe physical memory and exclude swapped-out memory. |
| VSZ, VIRT, or VmSize | Virtual address-space size, not the amount of physical RAM currently resident. | Do not use it alone to decide which process is consuming the most RAM. A process can reserve or map a large address space without touching all of it. |
| %MEM | A process memory percentage shown by the utility. | Check that utility’s documentation for the exact calculation and denominator; do not assume the field is identical across tools. |
RSS is usually the best starting point because it is readily available. If several processes share libraries or other pages, PSS gives a more useful proportional allocation. The kernel notes that RSS-related accounting may be updated asynchronously and can be imprecise; reading smaps provides a more detailed, slower snapshot.
Inspect one process in more detail
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Read the summary fields
grep -E '^(Name|VmSize|VmRSS|VmSwap|RssAnon|RssFile|RssShmem):' /proc/1234/statusVmRSS summarizes resident memory, while VmSize is virtual size and VmSwap reports swapped memory. The RSS breakdown fields can help distinguish anonymous, file-backed, and shared-memory pages. The kernel documentation says /proc/<pid>/status shows nearly the same information available through ps, along with additional fields.
Get mapping-level detail
For a more detailed view of a process’s memory mappings, use:
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less /proc/1234/smapsWhere the kernel provides it, smaps_rollup offers aggregate mapping statistics without requiring you to read every mapping:
cat /proc/1234/smaps_rollupAvailability depends on kernel support, and detailed mapping inspection takes more work than reading the status summary. See the kernel’s /proc documentation for the documented fields and accounting caveats.
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Check whether the whole system is under memory pressure
A process ranking does not account for all system memory, including kernel use, caches, and shared-memory accounting. Check system-wide RAM and swap separately:
free -hfree obtains its figures from /proc/meminfo. Do not judge available headroom from the free column alone: Linux uses otherwise idle memory for caches, some of which can be reclaimed. The free manual explains the displayed fields, and the kernel /proc documentation describes meminfo.
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Which method should you use?
| Need | Use | Trade-off |
|---|---|---|
| Rank processes at one moment | ps -eo pid,comm,rss,%mem --sort=-rss |
Fast and simple, but it is a snapshot and ranks by RSS. |
| Observe processes as usage changes | top or htop |
Interactive and live; fields and sort controls vary by version. |
| Understand shared-page attribution | PSS or USS, using smem or smaps fields |
More informative for shared memory than RSS; detailed inspection can be slower. |
| Assess overall RAM and swap | free -h or /proc/meminfo |
System-wide context rather than a per-process ranking. |
For more background on the fields shown by ps, Red Hat’s RHEL 7 System Administrator’s Guide explains the ps aux columns, including RSS, VSZ, and %MEM. Its examples describe RHEL 7 and should not be taken as a guarantee about every current Linux distribution.
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