PC Slower Than It Used to Be?
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchTo find the biggest memory users quickly, run ps -eo pid,comm,rss,%mem --sort=-rss | head -20. It ranks processes by resident memory (RSS), showing each process ID, command name, RSS, and memory percentage. For a live view, use top or, if installed, htop. Treat the ranking as a starting point: RSS can count shared pages in more than one process, so it is not an exact bill of each app’s exclusive RAM use.
Choose the right view for the question
| Method | Best for | What to know |
|---|---|---|
top or htop |
Watching a changing workload live | Interactive process lists can be sorted by memory. Field names and sort controls vary by utility version and configuration. See the top(1) manual and the htop(1) manual. |
Sorted ps |
A quick point-in-time ranking | The command below sorts by RSS. This procps-style option may not be supported by every ps implementation. |
/proc/<PID>/status |
Checking memory fields for one known process | Shows a compact per-process summary; replace <PID> with the actual process ID. |
smaps, smaps_rollup, or smem |
Understanding shared-page attribution | More detailed inspection can take longer. Availability and permissions can vary. |
free -h |
Checking overall RAM and swap context | System-wide figures complement, rather than duplicate, a process ranking. |
Rank processes by resident memory
Run this in a terminal:
ps -eo pid,comm,rss,%mem --sort=-rss | head -20The columns are process ID (PID), command name (COMM), resident set size (RSS), and the utility’s memory percentage (%MEM). Sorting by -rss puts the largest RSS values first. RSS is commonly displayed in kilobytes by procps-style tools; check your installed ps manual if your output differs. The percentage’s precise calculation and denominator are utility-specific, so do not assume it uses a particular system-memory figure without checking that manual.
To watch processes as they change, start top or htop. Find the memory-related column and sort the list by it using the controls documented by your installed version. htop may offer fields such as RSS and PSS, but available fields depend on its version and build.
Read the memory columns correctly
RSS or RES: resident pages, including shared pages
RSS (also shown as RES in some displays) represents physical pages currently resident for a process. It is a useful first comparison, but some pages are shared with other processes and can appear in each process’s RSS. Adding RSS values therefore does not give an exact total of physical RAM used exclusively by those processes. The Linux kernel documentation for /proc explains that RSS accounting is asynchronous and may not be very precise.
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PSS and USS: account for sharing differently
Proportional set size (PSS) divides shared pages among the processes using them; unshared pages count fully for the process. Unique set size (USS) counts the memory not shared with other processes. These measures can make process-level comparisons more informative when shared libraries or other shared mappings matter. The smem(8) manual describes PSS and USS; these figures describe physical memory and do not include swapped-out memory.
VSZ, VIRT, and VmSize: virtual address space
VSZ (also called VIRT or VmSize in different tools) describes a process’s virtual address space, not the RAM currently resident. A process can reserve or map a large range without touching all of it. The kernel documentation illustrates this distinction with a process that allocates 1 GiB but touches only 300 MiB. A large VSZ alone is not proof that a process is consuming that much physical RAM.
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%MEM: a tool-specific percentage
%MEM is a percentage shown by process utilities, but consult the documentation for the utility and version you are using to confirm its exact calculation and denominator. It is useful for scanning a list, not a substitute for understanding the underlying memory fields.
Inspect one process in more detail
After identifying a PID, inspect its status summary. Replace 1234 with the process ID you found:
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grep -E '^(Name|VmSize|VmRSS|VmSwap|RssAnon|RssFile|RssShmem):' /proc/1234/statusThe Linux kernel documentation says this file “shows you nearly the same information you would get if you viewed it with the ps command.” The named fields give a compact view of virtual size, resident memory, swap, and resident-memory categories. For mapping-level detail, use:
less /proc/1234/smapsScanning smaps provides more precise mapping information but is slower than reading a summary. If your kernel exposes it, smaps_rollup aggregates the mapping statistics and is more convenient to read:
Rank #4
cat /proc/1234/smaps_rollupKernel support and access permissions determine whether these files are available to you. The kernel documentation covers the formats and accounting limitations in its /proc filesystem guide.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Check whether system memory is actually under pressure
A list of large processes does not by itself show whether the machine is short of memory. Check overall RAM and swap with:
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free -hfree reports system-wide information based on /proc/meminfo; its manual explains the fields. Do not use the free column alone as a verdict: Linux uses otherwise available memory for caches, some of which can be reclaimed. Look at the available-memory figure and swap alongside process activity. The kernel’s /proc documentation describes the system memory information exposed through /proc/meminfo.
Quick Recap
- One process has high RSS, but available memory remains ample: the process is a large resident user, but the ranking alone does not establish a system-wide problem.
- Several processes have high RSS: remember that shared pages can be counted in multiple RSS values; use PSS or mapping details if you need a less duplicated attribution.
- Available memory is low or swap use is notable: examine the live process list and system-wide figures together. A single snapshot identifies candidates, not the cause or duration of pressure.
Practical sequence
- Get a quick ranking: run
ps -eo pid,comm,rss,%mem --sort=-rss | head -20. - Watch a changing workload: open
toporhtopand sort by the available memory field. - Check system context: run
free -hand consider available memory and swap, not just the free column. - Investigate a specific PID: read
/proc/<PID>/status; usesmaps_rollupif supported, orsmapsfor per-mapping detail. - Refine shared-memory comparisons: use PSS, for example through
smem, when RSS double-counting could mislead your comparison.
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