[Lustre-discuss] Lustre buffer cache causes large system overhead.
roy.dragseth at uit.no
Thu Aug 22 06:51:32 PDT 2013
We have just discovered that a large buffer cache generated from traversing a
lustre file system will cause a significant system overhead for applications
with high memory demands. We have seen a 50% slowdown or worse for
applications. Even High Performance Linpack, that have no file IO whatsoever
is affected. The only remedy seems to be to empty the buffer cache from memory
by running "echo 3 > /proc/sys/vm/drop_caches"
Any hints on how to improve the situation is greatly appreciated.
Client: Dual socket Sandy Bridge, with 32GB ram and infiniband connection to
lustre server. CentOS 6.4, with kernel 2.6.32-358.11.1.el6.x86_64 and lustre
v2.1.6 rpms downloaded from whamcloud download site.
Lustre: 1 MDS and 4 OSS running Lustre 2.1.3 (also from whamcloud site). Each
OSS has 12 OST, total 1.1 PB storage.
How to reproduce:
Traverse the lustre file system until the buffer cache is large enough. In our
case we run
find . -print0 -type f | xargs -0 cat > /dev/null
on the client until the buffer cache reaches ~15-20GB. (The lustre file system
has lots of small files so this takes up to an hour.)
Kill the find process and start a single node parallel application, we use HPL
(high performance linpack). We run on all 16 cores on the system with 1GB ram
per core (a normal run should complete in appr. 150 seconds.) The system
monitoring shows a 10-20% system cpu overhead and the HPL run takes more than
200 secs. After running "echo 3 > /proc/sys/vm/drop_caches" the system
performance goes back to normal with a run time at 150 secs.
I've created an infographic from our ganglia graphs for the above scenario.
Attached is an excerpt from perf top indicating that the kernel routine taking
the most time is _spin_lock_irqsave if that means anything to anyone.
It does not seem to matter if we mount lustre over infiniband or ethernet.
Filling the buffer cache with files from an NFS filesystem does not degrade
Filling the buffer cache with one large file does not give degraded performance.
(tested with iozone)
Again, any hints on how to proceed is greatly appreciated.
The Computer Center, University of Tromsø, N-9037 TROMSØ Norway.
phone:+47 77 64 41 07, fax:+47 77 64 41 00
Roy Dragseth, Team Leader, High Performance Computing
Direct call: +47 77 64 62 56. email: roy.dragseth at uit.no
-------------- next part --------------
Samples: 6M of event 'cycles', Event count (approx.): 634546877255
62.19% libmkl_avx.so [.] mkl_blas_avx_dgemm_kernel_0
13.30% mca_btl_sm.so [.] mca_btl_sm_component_progress
8.80% libmpi.so.1.0.3 [.] opal_progress
5.29% [kernel] [k] _spin_lock_irqsave
1.41% libmkl_avx.so [.] mkl_blas_avx_dgemm_copyan
1.17% mca_pml_ob1.so [.] mca_pml_ob1_progress
0.88% libmkl_avx.so [.] mkl_blas_avx_dtrsm_ker_ruu_a4_b8
0.41% [kernel] [k] compaction_alloc
0.38% [kernel] [k] _spin_lock_irq
0.36% mca_pml_ob1.so [.] opal_progress at plt
0.33% xhpl [.] HPL_dlaswp06T
0.28% libmkl_avx.so [.] mkl_blas_avx_dgemm_copybt
0.24% mca_pml_ob1.so [.] mca_pml_ob1_send
0.18% [kernel] [k] _spin_lock
0.17% [kernel] [k] __mem_cgroup_commit_charge
0.16% [kernel] [k] mem_cgroup_lru_del_list
0.16% [kernel] [k] putback_lru_page
0.16% [kernel] [k] __mem_cgroup_uncharge_common
0.15% xhpl [.] HPL_dlatcpy
0.15% xhpl [.] HPL_dlaswp01T
0.15% [kernel] [k] clear_page_c
0.15% xhpl [.] HPL_dlaswp10N
0.13% [kernel] [k] list_del
0.13% [kernel] [k] free_hot_cold_page
0.13% [kernel] [k] free_pcppages_bulk
0.13% [kernel] [k] release_pages
0.13% mca_pml_ob1.so [.] mca_pml_ob1_recv
0.12% [kernel] [k] ____pagevec_lru_add
0.12% [kernel] [k] copy_user_generic_string
0.12% [kernel] [k] compact_zone
0.10% xhpl [.] __intel_ssse3_rep_memcpy
0.10% [kernel] [k] __list_add
0.10% [kernel] [k] lookup_page_cgroup
0.09% [kernel] [k] mem_cgroup_end_migration
0.08% [kernel] [k] mem_cgroup_prepare_migration
0.08% [kernel] [k] get_pageblock_flags_group
0.08% [kernel] [k] page_waitqueue
0.07% [kernel] [k] migrate_pages
0.07% [kernel] [k] __wake_up_bit
0.07% [kernel] [k] get_page
0.07% [kernel] [k] unlock_page
0.07% [kernel] [k] mem_cgroup_lru_add_list
0.06% [kernel] [k] page_fault
0.06% [kernel] [k] __alloc_pages_nodemask
0.06% [kernel] [k] put_page
0.06% [kernel] [k] compact_checklock_irqsave
More information about the lustre-discuss