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From Daniel Biazus <daniel.bia...@azion.com>
Subject Interim cache - High CPU usage
Date Sun, 11 Jan 2015 20:52:06 GMT
Hi Guys,

We' ve been using ATS as a reverse proxy, and a few week ago We started to
use the interim cache feature in more intense way, caching objects with the
average size of 200 MB and max size of 1 GB.

*We have ~ 1 TB HDD as a default storage:*

cat /etc/trafficserver/storage.config

# ATS - Storage
/dev/sda6 volume=1

*And also, a 120 GB SDD storage as interim cache:*

LOCAL proxy.config.cache.interim.storage STRING /dev/sdc1

After 20 ~ 30 minutes in production with this configuration, We could
notice a sudden CPU high usage, increasing up to 65 %, considering that our
regular usage is 10 %. However the throughput still stable in 250 Mbps per
box.

We've found the following behavior using the perf top tool:

   88.18%  traffic_server      [.]
_Z15write_to_net_ioP10NetHandlerP18UnixNetVConnectionP7EThread
     0.32%  traffic_server      [.] _ZN10NetHandler12mainNetEventEiP5Event
     0.30%  [kernel]             [k] update_sd_lb_stats
     0.29%  [e1000e]           [k] e1000e_check_ltr_demote
     0.25%  [kernel]             [k] __ticket_spin_lock
     0.24%  traffic_server      [.] _ZN7EThread13process_eventEP5Eventi
     0.21%  [kernel]             [k] timerqueue_add
     0.17%  libc-2.12.so       [.] epoll_wait
     0.17%  libpcre.so.0.0.   [.] 0x00000000000100dd
     0.14%  [kernel]             [k] __schedule

1) This behavior is easily reproduced caching* large objects with interim
cache active*.
2) With interim cache *disabled*, this behavior* is not reproduced.*

 As you can see, at the perf top output, the write_to_net_*io *function is
responsible for this heavy CPU usage. We would like to hear of you guys, if
anyone has faced a issue like that, or if you have any clues about this
possible bug.

Thanks & Regards,

-- 

Daniel Biazus
infrastructure Engineering
Azion Technologies
Porto Alegre, Brasil +55 51 3012 3005 | +55 51 82279032
Miami, USA +1 305 704 8816

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