Jann Horn created COUCHDB-1761:
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Summary: Couchdb does lots of useless syscalls
Key: COUCHDB-1761
URL: https://issues.apache.org/jira/browse/COUCHDB-1761
Project: CouchDB
Issue Type: Bug
Components: Database Core
Reporter: Jann Horn
Try running a fresh couchdb with no replication jobs or so like this:
{code}strace -tt -f utils/run{code}
Don't connect to it or anything, just let it run. I'm seeing output like this:
{code}
[pid 18350] 17:40:15.086577 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.086610 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.086643 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.086675 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.086736 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.086771 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.086940 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.087180 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.087219 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.087251 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.087287 epoll_wait(3, {}, 256, 228) = 0
[pid 18350] 17:40:15.315646 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.315718 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.315751 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.315800 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.315833 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.315865 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.315900 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.315953 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.315992 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.316081 epoll_wait(3, {}, 256, 651) = 0
[pid 18350] 17:40:15.967979 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.969211 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.969926 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.970484 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.970616 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.970717 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.970806 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.970895 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.970983 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.971071 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.971158 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.971243 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:15.971343 epoll_wait(3, {}, 256, 345) = 0
[pid 18350] 17:40:16.316908 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.317266 epoll_wait(3, {}, 256, 650) = 0
[pid 18350] 17:40:16.969333 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.970057 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.970580 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.970736 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.970827 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.970917 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.971011 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.971118 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.971206 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.971300 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.971386 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:16.971485 epoll_wait(3, {}, 256, 115) = 0
[pid 18350] 17:40:17.087042 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:17.087421 accept(10, 0x7fb208dbfba0, [28]) = -1 EAGAIN
(Resource temporarily unavailable)
[pid 18350] 17:40:17.087915 epoll_ctl(3, EPOLL_CTL_DEL, 10, {EPOLLIN, {u32=10,
u64=73199780460757002}}) = 0
[pid 18350] 17:40:17.088035 epoll_ctl(3, EPOLL_CTL_ADD, 10, {EPOLLIN, {u32=10,
u64=73199780460757002}}) = 0
[pid 18350] 17:40:17.088139 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:17.088231 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:17.088327 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:17.088477 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:17.088565 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:17.088660 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:17.088746 epoll_wait(3, {}, 256, 0) = 0
[pid 18350] 17:40:17.088833 epoll_wait(3, {}, 256, 0) = 0
{code}
Why is couchdb calling epoll_wait so often with a timeout of 0? Why does it
remove an entry from the watched set, then immediately re-add it?
I have started looking at couchdb's syscalls because a couchdb instance on my
computer uses 2-4% CPU while it should be idle. It's not a big issue, but it
shouldn't be that way, I think.
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