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Additional clarification about %vmeff exceeding 100% in newer kernels #1782

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26 changes: 26 additions & 0 deletions xml/utilities.xml
Original file line number Diff line number Diff line change
Expand Up @@ -479,6 +479,32 @@ Average: 92.08 211.70 1097.30 0.26 12010.28 0.00 0.00 0
page swapped out is being reused) or 0 (no pages have been scanned).
The value should not drop below 30.
</para>
<note>
<title>Understanding and addressing <guimenu>%vmeff</guimenu> values exceeding 100% in modern Linux
kernels</title>
<para>
A <guimenu>%vmeff</guimenu> value exceeding 100% means that the number of pages reclaimed (<guimenu>pgsteal</guimenu>) is reported
as higher than the number of pages scanned (<guimenu>pgscank</guimenu>) during memory reclamation. This is
unexpected because under normal circumstances, the kernel should not reclaim more pages than it scans.
</para>
<para>
In modern Linux kernels (5.x and higher), changes in memory management and page reclamation
accounting make <guimenu>%vmeff</guimenu> less reliable as a measure of virtual memory efficiency. These
changes include separating background and direct reclaim metrics, tracking memory operations
with greater granularity, introducing deferred reclamation, and adding support for NUMA-aware
and cgroup-based memory management. Such updates can cause tools like <command>sar</command>
to misinterpret metrics like <guimenu>pgsteal</guimenu> and <guimenu>pgscank</guimenu>, resulting in <guimenu>%vmeff</guimenu> value
exceeding 100% in certain situations.
</para>
<para>
If <guimenu>%vmeff</guimenu> exceeds 100%, avoid using it as a performance indicator. Instead, analyze memory
performance by monitoring <guimenu>pgpgin/s</guimenu> and <guimenu>pgpgout/s</guimenu> for paging activity, <guimenu>majflt/s</guimenu> and <guimenu>fault/s</guimenu>
for page faults, and memory utilization details in <filename>/proc/meminfo</filename> (for
example, <guimenu>Active</guimenu>, <guimenu>Inactive</guimenu>, <guimenu>Dirty</guimenu> and <guimenu>Writeback</guimenu>). Additionally, correlate these metrics with
application-level performance and I/O patterns to identify potential memory bottlenecks or
inefficiencies.
</para>
</note>
</sect4>
<sect4 xml:id="sec-util-multi-sar-report-disk">
<title>Block device statistics report: <command>sar</command> <option>-d</option></title>
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