Lustre 1.6 Operations Manual

21.1.0.1 OSS Service Thread Count
The oss_num_threads parameter allows the number of OST service threads to be
specified at module load time on the OSS nodes:
options ost oss_num_threads={N}
An OSS can have a maximum of 512 service threads and a minimum of 2 service
threads. The number of service threads is a function of how much RAM and how
many CPUs are on each OSS node (1 thread / 128MB * num_cpus). If the load on the
OSS node is high, new service threads will be started in order to process more
requests concurrently, up to 4x the initial number of threads (subject to the
maximum of 512). For a 2GB 2-CPU system the default thread count is 32 and the
maximum thread count is 128.
Increasing the size of the thread pool may help when:
■ Several OSTs are exported from a single OSS
■ Back-end storage is running synchronously
■
I/O completions take excessive time
In such cases, a larger number of I/O threads allows the kernel and storage to
aggregate many writes together for more efficient disk I/O. The OSS thread pool is
shared—each thread allocates approximately 1.5 MB (maximum RPC size + 0.5 MB)
for internal I/O buffers.
Note – Consider memory consumption when increasing the thread pool size.
21.1.1 MDS Threads
There is a similar parameter for the number of MDS service threads:
options mds mds_num_threads={N}
At this time, we have not tested to determine the optimal number of MDS threads.
The default value varies, based on server size, up to a maximum of 32. The
maximum number of threads (MDS_MAX_THREADS) is 512.
Note – The OSS and MDS automatically start new service threads dynamically in
response to server loading within a factor of 4. The default is calculated the same
way as before (as explained in OSS Service Thread Count).
Setting the *_num_threads module parameter disables the automatic thread
creation behavior.
21-2 Lustre 1.6 Operations Manual • September 2008