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This is a rewrite of the hashmap implementation. Its advantage is lower
memory usage.
It uses open addressing (entries are stored in an array, as opposed to
linked lists). Hash collisions are resolved with linear probing and
Robin Hood displacement policy. See the references in hashmap.c.
Some fun empirical findings about hashmap usage in systemd on my laptop:
- 98 % of allocated hashmaps are Sets.
- Sets contain 78 % of all entries, plain Hashmaps 17 %, and
OrderedHashmaps 5 %.
- 60 % of allocated hashmaps contain only 1 entry.
- 90 % of allocated hashmaps contain 5 or fewer entries.
- 75 % of all entries are in hashmaps that use trivial_hash_ops.
Clearly it makes sense to:
- store entries in distinct entry types. Especially for Sets - their
entries are the most numerous and they require the least information
to store an entry.
- have a way to store small numbers of entries directly in the hashmap
structs, and only allocate the usual entry arrays when the direct
storage is full.
The implementation has an optional debugging feature (enabled by
defining the ENABLE_HASHMAP_DEBUG macro), where it:
- tracks all allocated hashmaps in a linked list so that one can
easily find them in gdb,
- tracks which function/line allocated a given hashmap, and
- checks for invalid mixing of hashmap iteration and modification.
Since entries are not allocated one-by-one anymore, mempools are not
used for entries. Originally I meant to drop mempools entirely, but it's
still worth it to use them for the hashmap structs. My testing indicates
that it makes loading of units about 5 % faster (a test with 10000 units
where more than 200000 hashmaps are allocated - pure malloc: 449±4 ms,
mempools: 427±7 ms).
Here are some memory usage numbers, taken on my laptop with a more or
less normal Fedora setup after booting with SELinux disabled (SELinux
increases systemd's memory usage significantly):
systemd (PID 1) Original New Change
dirty memory (from pmap -x 1) [KiB] 2152 1264 -41 %
total heap allocations (from gdb-heap) [KiB] 1623 756 -53 %
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It cannot fail in the current hashmap implementation, but it may fail in
alternative implementations (unless a sufficiently large reservation has
been placed beforehand).
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With the current hashmap implementation that uses chaining, placing a
reservation can serve two purposes:
- To optimize putting of entries if the number of entries to put is
known. The reservation allocates buckets, so later resizing can be
avoided.
- To avoid having very long bucket chains after using
hashmap_move(_one).
In an alternative hashmap implementation it will serve an additional
purpose:
- To guarantee a subsequent hashmap_move(_one) will not fail with
-ENOMEM (this never happens in the current implementation).
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The following hashmap_* and set_* functions/macros have never had any
users in systemd's history:
*_iterate_backwards
*_iterate_skip
*_last
*_FOREACH_BACKWARDS
Remove this dead code.
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It is redundant to store 'hash' and 'compare' function pointers in
struct Hashmap separately. The functions always comprise a pair.
Store a single pointer to struct hash_ops instead.
systemd keeps hundreds of hashmaps, so this saves a little bit of
memory.
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Freeing in error path is the common pattern with set_put().
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The argument given to the __attribute__((cleanup)) functions is the
address of the variable that's going out of scope. It cannot be NULL.
The "if (!s)" check in set_freep() is pointless.
Perhaps "if (!*s)" was intented. But that's pointless too, because
set_free()/set_free_free() are OK to call with a NULL argument (just
like free()).
Setting "*s = NULL" is pointless, because the variable that s points
to is about to go out of scope.
The same holds for strv_freep().
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set_freep() is added to automatize set_free().
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In some cases, like wrong configuration, restarting after error
does not help, so administrator can specify statuses by RestartPreventExitStatus
which will not cause restart of a service.
Sometimes you have non-standart exit status, so this can be specified
by SuccessfulExitStatus.
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We finally got the OK from all contributors with non-trivial commits to
relicense systemd from GPL2+ to LGPL2.1+.
Some udev bits continue to be GPL2+ for now, but we are looking into
relicensing them too, to allow free copy/paste of all code within
systemd.
The bits that used to be MIT continue to be MIT.
The big benefit of the relicensing is that closed source code may now
link against libsystemd-login.so and friends.
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internal libraries
Before:
$ ldd /lib/systemd/systemd-timestamp
linux-vdso.so.1 => (0x00007fffb05ff000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f90aac57000)
libcap.so.2 => /lib64/libcap.so.2 (0x00007f90aaa53000)
librt.so.1 => /lib64/librt.so.1 (0x00007f90aa84a000)
libc.so.6 => /lib64/libc.so.6 (0x00007f90aa494000)
/lib64/ld-linux-x86-64.so.2 (0x00007f90aae90000)
libdl.so.2 => /lib64/libdl.so.2 (0x00007f90aa290000)
libattr.so.1 => /lib64/libattr.so.1 (0x00007f90aa08a000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f90a9e6e000)
After:
$ ldd systemd-timestamp
linux-vdso.so.1 => (0x00007fff3cbff000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f5eaa1c3000)
librt.so.1 => /lib64/librt.so.1 (0x00007f5ea9fbb000)
libc.so.6 => /lib64/libc.so.6 (0x00007f5ea9c04000)
/lib64/ld-linux-x86-64.so.2 (0x00007f5eaa3fc000)
libdl.so.2 => /lib64/libdl.so.2 (0x00007f5ea9a00000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f5ea97e4000)
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