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| core: implement a global thread pool
This patch implements a thread pool that is wait-free for adding jobs to
the queue and uses a very small locked region to get jobs. This makes it
possible to decrease contention drastically. It's based on wfcqueue
structure provided by urcu library.
It automatically enables more threads when load demands it, and stops
them when not needed. There's a maximum number of threads that can be
used. This value can be configured.
Depending on the workload, the maximum number of threads plays an
important role. So it needs to be configured for optimal performance.
Currently the thread pool doesn't self adjust the maximum for the
workload, so this configuration needs to be changed manually.
For this reason, the global thread pool has been made optional, so that
volumes can still use the thread pool provided by io-threads.
To enable it for bricks, the following option needs to be set:
config.global-threading = on
This option has no effect if bricks are already running. A restart is
required to activate it. It's recommended to also enable the following
option when running bricks with the global thread pool:
performance.iot-pass-through = on
To enable it for a FUSE mount point, the option '--global-threading'
must be added to the mount command. To change it, an umount and remount
is needed. It's recommended to disable the following option when using
global threading on a mount point:
performance.client-io-threads = off
Currently it can only be enabled for bricks and FUSE mounts.
The maximum number of threads for clients and bricks can be configured
using the following options:
config.client-threads
config.brick-threads
These options can be applied online and its effect is immediate most of
the times. If one of them is set to 0, the maximum number of threads
will be calcutated as #cores * 2.
Some distributions use a very old userspace-rcu library (version 0.7)
for this reason, some header files from version 0.10 have been copied
into contrib/userspace-rcu and are used if the detected version is 0.7
or older.
If you also want to enable global-threading for fuse clients, you also need to set this option:
# gluster volume set <volname> performance.client-io-threads off
And remount the volume using --global-threading mount option.
gluster volume set dht_vol performance.client-io-threads off
mount -t glusterfs -o acl,global-threading 127.0.0.1:/dht_vol /mnt/data
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