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man: make clear that sd-journal notifications always come with extra latency

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Replaces: #17699
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Lennart Poettering 2021-02-16 17:15:16 +01:00
parent a9a43d8aa2
commit 696e5a8a73
1 changed files with 12 additions and 12 deletions
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24
man/sd_journal_get_fd.xml
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@ -151,18 +151,18 @@ else {
<function>poll()</function> and
<function>sd_journal_process()</function> into one.</para>
<para><function>sd_journal_reliable_fd()</function> may be used to
check whether the wakeup events from the file descriptor returned
by <function>sd_journal_get_fd()</function> are known to be
immediately triggered. On certain file systems where file change
events from the OS are not available (such as NFS) changes need to
be polled for repeatedly, and hence are detected only with a
certain latency. This call will return a positive value if the
journal changes are detected immediately and zero when they need
to be polled for and hence might be noticed only with a certain
latency. Note that there is usually no need to invoke this function
directly as <function>sd_journal_get_timeout()</function> on these
file systems will ask for timeouts explicitly anyway.</para>
<para><function>sd_journal_reliable_fd()</function> may be used to check whether the wake-up events from
the file descriptor returned by <function>sd_journal_get_fd()</function> are known to be quickly
triggered. On certain file systems where file change events from the OS are not available (such as NFS)
changes need to be polled for repeatedly, and hence are detected only with a considerable latency. This
call will return a positive value if the journal changes are detected quickly and zero when they need to
be polled for. Note that there is usually no need to invoke this function directly as
<function>sd_journal_get_timeout()</function> will request appropriate timeouts anyway.</para>
<para>Note that all of the above change notification interfaces do not report changes
instantly. Latencies are introduced for multiple reasons: as mentioned certain storage backends require
time-based polling, in other cases wake-ups are optimized by coalescing events, and the OS introduces
additional IO/CPU scheduling latencies.</para>
</refsect1>
<refsect1>