[Ur] TechEmpower Benchmarks

[escalier at riseup.net]

> I expect it will immediately reduce dramatically the number of "errors"
reported in the benchmark results.

I rebuilt the benchmark with the HEAD version of Ur/Web. Unfortunately, it
doesn't seem to have done much for the socket errors.

`./wrk -d 5 -c 10 -t 10 'http://127.0.0.1:8080/json'`

Running 5s test @ http://127.0.0.1:8080/json
  10 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     1.32ms    0.90ms   3.60ms   85.64%
    Req/Sec     0.90k   391.67     1.44k    79.53%
  44289 requests in 6.00s, 7.98MB read
  Socket errors: connect 0, read 44281, write 0, timeout 2
Requests/sec:   7386.89
Transfer/sec:      1.33MB

> escalier, do you know if there's a way to request a higher value of the
configuration setting mentioned in the hint?

When I set max_pred_locks_per_transaction to 256 in my local
postgresql.conf (from the default of 64), those errors seem to go away.
I've opened an issue for that
(https://github.com/TechEmpower/FrameworkBenchmarks/issues/632).