[systemd-devel] Is sd_journal_send non-blocking?

[Florian Weimer]

On 02/06/2017 12:36 PM, Lennart Poettering wrote:
> On Mon, 06.02.17 11:12, Florian Weimer (fweimer at redhat.com) wrote:
>
>> The manual page I've got does not say it is, but the implementation does not
>> wait for acknowledgment from the journal.  The implementation does not block
>> explicitly, but I think the sendmsg call can block until the receiver queue
>> is empty.
>>
>> The background for this question is that we have a feature request for a
>> non-blocking logging interface.  I wonder if the journal fits this
>> requirement.
>>
>> I assume that with “non-blocking”, the feature request submitter means that
>> the function does not block indefinitely, say due to a service process
>> outage.  Memory allocations can take a fairly long time as well (due to
>> paging), but I think that doesn't count here.
>
> The socket we use is blocking, but we try to increase the socket
> buffer to 8M, to move the point where we start to block out late. But
> that only works with sufficient privileges.

The sender buffer size is currently a limit on the datagram size.  The 
data goes straight into the receiver's buffer, so it doesn't have any 
impact on blocking behavior.

> So yes, we are always blocking, we don't throw data away.

Sure.  I was wondering if indefinite blocking is considered a critical 
service failure and if there is a watchdog which would catch a stuck 
journal daemon.

(Hanging syslog servers are apparently a fairly common problem, and this 
is where the RFE originally came from.)

> I'd be willing to take a patch however, that adds a call
> sd_journal_set_block_timeout() or so, that takes a time value we pass
> to SO_SNDTIMEO for the logging socket. This would permit clients to
> precisely control how long they want us to wait before we give up. And
> in the case where a zero timeout is set we'd instead set O_NONBLOCK,
> thus making logging entirely non-blocking.

This would help with discarding data.  It would not help those who want 
to integrate logging into an event loop.  Which is probably bad idea 
anyway, but I'm not quite sure yet what the purpose of non-blocking 
logging is.  Perhaps there is a desire to apply some back-pressure to 
reduce the rate at which logging messages are generated.  But in 
general, this merely introduces deadlocks.

Thanks,
Florian