Here are the SELECT only pgbench test results from my E5-2620 machine,
with HT on and off:
HT off:
bash-4.1$ /usr/pgsql-9.2/bin/pgbench -T 600 -j 48 -c 48 -S
starting vacuum...end.
transaction type: SELECT only
scaling factor: 100
query mode: simple
number of clients: 48
number of threads: 48
duration: 600 s
number of transactions actually processed: 25969680
tps = 43281.392273 (including connections establishing)
tps = 43282.476955 (excluding connections establishing)
All 6 cores saturated:
avg-cpu: %user %nice %system %iowait %steal %idle
81.42 0.00 18.21 0.00 0.00 0.37
HT on:
bash-4.1$ /usr/pgsql-9.2/bin/pgbench -T 600 -j 48 -c 48 -S
starting vacuum...end.
transaction type: SELECT only
scaling factor: 100
query mode: simple
number of clients: 48
number of threads: 48
duration: 600 s
number of transactions actually processed: 29934601
tps = 49888.697225 (including connections establishing)
tps = 49889.570754 (excluding connections establishing)
12% of CPU showing as idle (whether that's true or not I'm not sure):
avg-cpu: %user %nice %system %iowait %steal %idle
71.09 0.00 16.99 0.00 0.00 11.92
So for this particular test HT gives us the equivalent of about one
extra core.
It does not reduce performance, rather increases performance slightly.
