Stuck with sc_rx_timeout

[info at dubistmeinheld.de]

Hi,

I'm closely monitoring varnish 5.0 stats. I could fix some issues, but I
am stuck on where sc_rx_timeout is coming from.

Looking at the documentation, it says "Number of session closes with
Error RX_TIMEOUT (Receive timeout)".

I do not fully understand this sentence.
- Is this a timeout expressing that varnish does not receive an answer
within a certain time from the backend?
- Do these timeouts happen from time to time and are ok, or is there an
issue on the server (code, params?).
- If parameters, which ones could be candidates to tune?
- Also I have issues with more parameters like sc_req_http10 (please
have a look below) and unsure if they are severe.

I would be happy if you could point me in some direction.

Cheers,
Jens

# /usr/sbin/varnishstat -1 | grep -i '\(err\|fail\|drop\)'
MAIN.sess_drop               0         0.00 Sessions dropped
MAIN.sess_fail               0         0.00 Session accept failures
MAIN.client_req_400            2         0.00 Client requests received,
subject to 400 errors
MAIN.client_req_417            0         0.00 Client requests received,
subject to 417 errors
MAIN.backend_fail                 2         0.00 Backend conn. failures
MAIN.fetch_failed                 0         0.00 Fetch failed (all causes)
MAIN.fetch_no_thread              0         0.00 Fetch failed (no thread)
MAIN.threads_failed               0         0.00 Thread creation failed
MAIN.sess_dropped                 0         0.00 Sessions dropped for thread
MAIN.sess_closed_err          21784         0.30 Session Closed with error
MAIN.sc_req_http10             1223         0.02 Session Err REQ_HTTP10
MAIN.sc_rx_bad                    0         0.00 Session Err RX_BAD
MAIN.sc_rx_body                  34         0.00 Session Err RX_BODY
MAIN.sc_rx_junk                   2         0.00 Session Err RX_JUNK
MAIN.sc_rx_overflow               0         0.00 Session Err RX_OVERFLOW
MAIN.sc_rx_timeout            20525         0.29 Session Err RX_TIMEOUT
MAIN.sc_tx_error                  0         0.00 Session Err TX_ERROR
MAIN.sc_overload                  0         0.00 Session Err OVERLOAD
MAIN.sc_pipe_overflow             0         0.00 Session Err PIPE_OVERFLOW
MAIN.sc_range_short               0         0.00 Session Err RANGE_SHORT
MAIN.esi_errors                              0         0.00 ESI parse
errors (unlock)
SMA.s0.c_fail                                0         0.00 Allocator
failures
SMA.Transient.c_fail                         0         0.00 Allocator
failures