-with-rtsopts=-T results in severe slowdown

[jkachmar]

Just ran a few tests on a small Servant app and got these results:

# with -with-rtsopts=-T
Lifting the server siege...
Transactions:		        2131 hits
Availability:		      100.00 %
Elapsed time:		       19.58 secs
Data transferred:	        0.05 MB
Response time:		        0.45 secs
Transaction rate:	      108.84 trans/sec
Throughput:		        0.00 MB/sec
Concurrency:		       49.41
Successful transactions:        2131
Failed transactions:	           0
Longest transaction:	        0.60
Shortest transaction:	        0.02

# without -with-rtsopts=-T
Lifting the server siege...
Transactions:		        4699 hits
Availability:		      100.00 %
Elapsed time:		       19.73 secs
Data transferred:	        0.10 MB
Response time:		        0.21 secs
Transaction rate:	      238.17 trans/sec
Throughput:		        0.01 MB/sec
Concurrency:		       49.67
Successful transactions:        4699
Failed transactions:	           0
Longest transaction:	        0.31
Shortest transaction:	        0.03

Haven't looked into it too much closer, but the setup between the two is entirely identical except for the -with-rtsopts=-T arg.

I'll try to extract something out into a more concrete repro, but I was pretty surprised when I saw it as the README suggests that the option should be pretty innocuous.

[23Skidoo]

This is probably more apprioriate for the GHC bug tracker.

[nh2]

Was any ghc bug filed for this?

[jkachmar]

Apologies, but no.

I never got around to extracting a small reproduction and didn’t feel comfortable submitting a bug report to the GHC tracker without one.