A call to sd_event_source_set_io_events() skipps calling into the kernel
if the new event-mask matches the old one. This is safe for
level-triggered sources as the kernel moves them onto the ready-list
automatically if events change. However, edge-triggered sources might not
be on the ready-list even though events are present.
A call to sd_event_source_set_io_events() with EPOLLET set might thus be
used to just move the io-source onto the ready-list so the next poll
will return it again. This is very useful to avoid starvation in
priority-based event queues.
Imagine a read() loop on an edge-triggered fd. If we cannot read data fast
enough to drain the receive queue, we might decide to skip reading for now
and schedule it for later. On edge-triggered io-sources we have to make
sure it's put on the ready-list so the next dispatch-round will return it
again if it's still the highest priority task. We could make sd-event
handle edge-triggered sources directly and allow marking them ready again.
However, it's much simpler to let the kernel do that for now via
EPOLL_CTL_MOD.
The check only cares about whether the module is installed, not enabled.
But installation we should know anyway, after all we ship the module
with systemd these days...
An administrator might want to block a certain sysusers config file from
being executed, e.g. to block the creation of a certain user.
Only a relatively short description is added in the man page, since
overrides should be relatively rare.
Currently after exiting rescue shell we isolate default target. User
might want to isolate to some other target than default one. However
issuing systemctl isolate command to desired target would bring system
to default target as a consequence of running ExecStopPost action.
Having common ancestor for rescue shell and possible followup systemctl
default command should fix this. If user exits rescue shell we will
proceed with isolating default target, otherwise, on manual isolate,
parent shell process is terminated and we don't isolate default target,
but target chosen by user.
Suggested-by: Michal Schmidt <mschmidt@redhat.com>
The new lzma2 compression options at the top of compress_blob_xz are
equivalent to using preset "0", exept for using a 1 MiB dictionary
(the same as preset "1"). This makes the memory usage at most 7.5 MiB
in the compressor, and 1 MiB in the decompressor, instead of the
previous 92 MiB in the compressor and 8 MiB in the decompressor.
According to test-compress-benchmark this commit makes XZ compression
20 times faster, with no increase in compressed data size.
Using more realistic test data (an ELF binary rather than repeating
ASCII letters 'a' through 'z' in order) it only provides a factor 10
speedup, and at a cost if a 10% increase in compressed data size.
But that is still a worthwhile trade-off.
According to test-compress-benchmark XZ compression is still 25 times
slower than LZ4, but the compressed data is one eighth the size.
Using more realistic test data XZ compression is only 18 times slower
than LZ4, and the compressed data is only one quarter the size.
$ ./test-compress-benchmark
XZ: compressed & decompressed 2535300963 bytes in 42.30s (57.15MiB/s), mean compresion 99.95%, skipped 3570 bytes
LZ4: compressed & decompressed 2535303543 bytes in 1.60s (1510.60MiB/s), mean compresion 99.60%, skipped 990 bytes
The systemd-escape utility might be used during early boot (e.g. when
being triggered from udev rules), so move it to rootbindir to support
systems with a split /usr setup.
Instead of waiting for new data from the sensor, which might be
a long time coming, depending on the sensor device, ask the kernel
for the last state for that particular input device.
