[systemd-devel] systemd-resolved: performance question

[Robert Ayrapetyan]

Hello, I'm using systemd-resolved on the server which performs a lot
of DNS queries (~20K per second) and systemd-resolved helps a lot
providing a cache:

Cache
  Current Cache Size: 263
          Cache Hits: 30928976
          Cache Misses: 2961

However, systemd-resolved process almost constantly utilizes 100% of
CPU (I don't have any other dns services like dnsmasq installed).

strace shows this:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 48.73    0.966765           4    194141           stat
 15.06    0.298804          12     24106           sendmsg
 13.59    0.269703           8     32346     32346 openat
  6.42    0.127381           5     24118           recvmsg
  5.49    0.108829           4     24118           recvfrom
  5.34    0.106037           4     24118           epoll_wait
  5.33    0.105838           4     24118           gettid
  0.01    0.000151           6        24           epoll_ctl

And in particular for stat, it queries just 3 files in a loop:

stat("/etc/resolv.conf", {st_mode=S_IFREG|0644, st_size=738, ...}) = 0
<0.000010>
stat("/run/systemd/resolve/resolv.conf", {st_mode=S_IFREG|0644,
st_size=624, ...}) = 0 <0.000009>
stat("/run/systemd/resolve/stub-resolv.conf", {st_mode=S_IFREG|0644,
st_size=738, ...}) = 0 <0.000013>

My question is: why does it "stats" so often? Is this a result of my
misconfiguration?

/etc/systemd/resolved.conf:
...
FallbackDNS=1.1.1.1 1.0.0.1

/etc/nsswitch.conf:
...
hosts:          resolve [!UNAVAIL=return] files dns