Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Aug 30, 2011 at 6:33 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I ran it up to "pgbench -c 200 -j 200 -S -T 300 bench" and still see
>> vmstat numbers around 50% user time, 12% system time, 38% idle.
>> So no lseek problem here, boss. Kernel calls itself 2.6.32-192.el6.x86_64.
> Eh, wait a minute. 38% idle time? Did you use a scale factor that
> doesn't fit in shared_buffers?
Nope: -s 100, 8GB shared_buffers, same as all the other tests.
Typical strace of one backend looks like
recvfrom(9, "Q\0\0\0?SELECT abalance FROM pgbenc"..., 8192, 0, NULL, NULL) = 64
lseek(10, 0, SEEK_END) = 269213696
lseek(11, 0, SEEK_END) = 224641024
sendto(9, "T\0\0\0!\0\1abalance\0\0\0\241\267\0\3\0\0\0\27\0\4\377\377\377\377"..., 66, 0, NULL, 0) = 66
recvfrom(9, "Q\0\0\0?SELECT abalance FROM pgbenc"..., 8192, 0, NULL, NULL) = 64
lseek(10, 0, SEEK_END) = 269213696
lseek(11, 0, SEEK_END) = 224641024
sendto(9, "T\0\0\0!\0\1abalance\0\0\0\241\267\0\3\0\0\0\27\0\4\377\377\377\377"..., 66, 0, NULL, 0) = 66
recvfrom(9, "Q\0\0\0?SELECT abalance FROM pgbenc"..., 8192, 0, NULL, NULL) = 64
select(0, NULL, NULL, NULL, {0, 1000}) = 0 (Timeout)
lseek(10, 0, SEEK_END) = 269213696
lseek(11, 0, SEEK_END) = 224641024
select(0, NULL, NULL, NULL, {0, 1000}) = 0 (Timeout)
select(0, NULL, NULL, NULL, {0, 1000}) = 0 (Timeout)
sendto(9, "T\0\0\0!\0\1abalance\0\0\0\241\267\0\3\0\0\0\27\0\4\377\377\377\377"..., 66, 0, NULL, 0) = 66
recvfrom(9, "Q\0\0\0?SELECT abalance FROM pgbenc"..., 8192, 0, NULL, NULL) = 64
lseek(10, 0, SEEK_END) = 269213696
lseek(11, 0, SEEK_END) = 224641024
select(0, NULL, NULL, NULL, {0, 1000}) = 0 (Timeout)
sendto(9, "T\0\0\0!\0\1abalance\0\0\0\241\267\0\3\0\0\0\27\0\4\377\377\377\377"..., 66, 0, NULL, 0) = 66
No I/O anywhere. I'm thinking the reported idle time must correspond to
spinlock delays that are long enough to reach the select() calls in
s_lock. If so, 38% is depressingly high, but it's not out of line with
what we've seen in the past in tests designed to provoke spinlock
contention.
(BTW, this is with the unlocked test added to TAS_SPIN.)
regards, tom lane