Free dirty objects, page cache and reclaimable slab objects for troubleshooting or instructional purposes.
Free dirty objects
Use sync utility to synchronize memory waiting to be written back to the persistent storage.
$ sync
Inspect /proc/meminfo to observe that Dirty memory that is waiting to be written back to the disk (defined using kilobytes) will decrease. You can also notice a short surge of the Writeback memory which is actively being written back to the persistent storage and WritebackTmp which is directly related to FUSE (Filesystem in Userspace).
Changes in memory usage before and after issuing this command.
MemTotal: 16287088 kB ( MemFree: 7688696 kB | MemFree: 7713352 kB MemAvailable: 8315048 kB | MemAvailable: 8339800 kB Buffers: 18452 kB | Buffers: 18492 kB Cached: 1836208 kB | Cached: 1836268 kB SwapCached: 0 kB ( Active: 6457040 kB | Active: 6430504 kB Inactive: 1459164 kB | Inactive: 1459204 kB Active(anon): 6070180 kB | Active(anon): 6043584 kB Inactive(anon): 1021284 kB ( Active(file): 386860 kB | Active(file): 386920 kB Inactive(file): 437880 kB | Inactive(file): 437920 kB Unevictable: 32 kB ( Mlocked: 32 kB ( SwapTotal: 999420 kB ( SwapFree: 999420 kB ( Dirty: 21200 kB | Dirty: 0 kB ← Writeback: 0 kB ( AnonPages: 6061728 kB | AnonPages: 6035384 kB Mapped: 1248224 kB | Mapped: 1248260 kB Shmem: 1029928 kB ( Slab: 348728 kB | Slab: 348748 kB SReclaimable: 120864 kB | SReclaimable: 120852 kB SUnreclaim: 227864 kB | SUnreclaim: 227896 kB KernelStack: 29168 kB | KernelStack: 29184 kB PageTables: 91616 kB | PageTables: 91656 kB NFS_Unstable: 0 kB ( Bounce: 0 kB ( WritebackTmp: 0 kB ( CommitLimit: 9142964 kB ( Committed_AS: 22529316 kB | Committed_AS: 22536664 kB VmallocTotal: 34359738367 kB ( VmallocUsed: 0 kB ( VmallocChunk: 0 kB ( HardwareCorrupted: 0 kB ( AnonHugePages: 38912 kB ( ShmemHugePages: 0 kB ( ShmemPmdMapped: 0 kB ( CmaTotal: 0 kB ( CmaFree: 0 kB ( HugePages_Total: 0 ( HugePages_Free: 0 ( HugePages_Rsvd: 0 ( HugePages_Surp: 0 ( Hugepagesize: 2048 kB ( DirectMap4k: 658988 kB ( DirectMap2M: 11786240 kB ( DirectMap1G: 5242880 kB
Free page cache
Use sysctl utility to issue command to the kernel to invalidate page cache which is the main disk cache used when data is either read from disk or written to disk.
$ sudo sysctl -w vm.drop_caches=1
Inspect /proc/meminfo to observe that page cache in memory will decrease.
Changes in memory usage before and after issuing this command.
MemTotal: 16287088 kB ( MemFree: 1595736 kB | MemFree: 7079620 kB MemAvailable: 8339004 kB | MemAvailable: 8325900 kB Buffers: 1714248 kB | Buffers: 1088 kB ← Cached: 5524636 kB | Cached: 1853228 kB ← SwapCached: 0 kB ( Active: 8972220 kB | Active: 6319944 kB Inactive: 4226696 kB | Inactive: 1496236 kB Active(anon): 5963252 kB | Active(anon): 5962128 kB Inactive(anon): 1063684 kB | Inactive(anon): 1076596 kB Active(file): 3008968 kB | Active(file): 357816 kB ← Inactive(file): 3163012 kB | Inactive(file): 419640 kB ← Unevictable: 32 kB ( Mlocked: 32 kB ( SwapTotal: 999420 kB ( SwapFree: 999420 kB ( Dirty: 296 kB | Dirty: 968 kB Writeback: 0 kB ( AnonPages: 5955404 kB | AnonPages: 5957264 kB Mapped: 1250196 kB | Mapped: 1250348 kB Shmem: 1066912 kB | Shmem: 1076720 kB Slab: 1149756 kB | Slab: 1047280 kB SReclaimable: 914580 kB | SReclaimable: 812116 kB SUnreclaim: 235176 kB | SUnreclaim: 235164 kB KernelStack: 29200 kB | KernelStack: 29104 kB PageTables: 91272 kB | PageTables: 91288 kB NFS_Unstable: 0 kB ( Bounce: 0 kB ( WritebackTmp: 0 kB ( CommitLimit: 9142964 kB ( Committed_AS: 22527888 kB | Committed_AS: 22538996 kB VmallocTotal: 34359738367 kB ( VmallocUsed: 0 kB ( VmallocChunk: 0 kB ( HardwareCorrupted: 0 kB ( AnonHugePages: 38912 kB ( ShmemHugePages: 0 kB ( ShmemPmdMapped: 0 kB ( CmaTotal: 0 kB ( CmaFree: 0 kB ( HugePages_Total: 0 ( HugePages_Free: 0 ( HugePages_Rsvd: 0 ( HugePages_Surp: 0 ( Hugepagesize: 2048 kB ( DirectMap4k: 658988 kB ( DirectMap2M: 11786240 kB ( DirectMap1G: 5242880 kB (
Free reclaimable slab objects
Use sysctl utility to issue command to the kernel to free reclaimable slab objects including dentries and inodes.
$ sudo sysctl -w vm.drop_caches=2
Inspect /proc/meminfo to observe that SReclaimable memory allocated for internal kernel data structures that can be reclaimed will decrease.
Inspect /proc/slabinfo for detailed information on inode, dentry caches.
Changes in memory usage before and after issuing this command.
MemTotal: 16287088 kB ( MemFree: 7078272 kB | MemFree: 7827464 kB MemAvailable: 8329644 kB | MemAvailable: 8411084 kB Buffers: 2564 kB | Buffers: 3696 kB Cached: 1843068 kB | Cached: 1804908 kB SwapCached: 0 kB ( Active: 6334520 kB | Active: 6341572 kB Inactive: 1484256 kB | Inactive: 1438124 kB Active(anon): 5973268 kB | Active(anon): 5979840 kB Inactive(anon): 1062816 kB | Inactive(anon): 1016812 kB Active(file): 361252 kB | Active(file): 361732 kB Inactive(file): 421440 kB | Inactive(file): 421312 kB Unevictable: 32 kB ( Mlocked: 32 kB ( SwapTotal: 999420 kB ( SwapFree: 999420 kB ( Dirty: 1100 kB | Dirty: 1568 kB Writeback: 0 kB ( AnonPages: 5968564 kB | AnonPages: 5971180 kB Mapped: 1249996 kB | Mapped: 1248236 kB Shmem: 1062948 kB | Shmem: 1025484 kB Slab: 1047264 kB | Slab: 346196 kB SReclaimable: 811972 kB | SReclaimable: 118788 kB ← SUnreclaim: 235292 kB | SUnreclaim: 227408 kB KernelStack: 29120 kB | KernelStack: 29136 kB PageTables: 91416 kB | PageTables: 91560 kB NFS_Unstable: 0 kB ( Bounce: 0 kB ( WritebackTmp: 0 kB ( CommitLimit: 9142964 kB ( Committed_AS: 22508796 kB | Committed_AS: 22493648 kB VmallocTotal: 34359738367 kB ( VmallocUsed: 0 kB ( VmallocChunk: 0 kB ( HardwareCorrupted: 0 kB ( AnonHugePages: 38912 kB ( ShmemHugePages: 0 kB ( ShmemPmdMapped: 0 kB ( CmaTotal: 0 kB ( CmaFree: 0 kB ( HugePages_Total: 0 ( HugePages_Free: 0 ( HugePages_Rsvd: 0 ( HugePages_Surp: 0 ( Hugepagesize: 2048 kB ( DirectMap4k: 658988 kB ( DirectMap2M: 11786240 kB ( DirectMap1G: 5242880 kB (
Changes in kernel slab allocator statistics before and after issuing this command.
$ sdiff -l -w 57 slabinfo.20 slabinfo.21 slabinfo - version: 2.1 ( # name <active ( nfs_direct_cache 0 ( nfs_read_data 68 ( nfs_inode_cache 102 | nfs_inode_cache 100 ← fscache_cookie_jar 2 ( nf_conntrack 1727 | nf_conntrack 1752 ovl_inode 43303 | ovl_inode 3096 ← au_finfo 0 ( au_icntnr 0 ( au_dinfo 0 ( kvm_async_pf 0 ( kvm_vcpu 0 ( kvm_mmu_page_header ( pte_list_desc 0 ( rpc_inode_cache 41 ( ext4_groupinfo_4k 7454 ( drm_i915_gem_request 9 ( i915_vma 2954 | i915_vma 2953 drm_i915_gem_object 292 | drm_i915_gem_object 292 RAWv6 464 ( UDPv6 260 ( tw_sock_TCPv6 34 ( request_sock_TCPv6 0 ( TCPv6 135 ( kcopyd_job 0 ( dm_uevent 0 ( cfq_io_cq 476 ( cfq_queue 510 ( bsg_cmd 0 ( mqueue_inode_cache 62 ( fuse_request 67 ( fuse_inode 98 | fuse_inode 76 ← ecryptfs_inode_cache ( ecryptfs_auth_tok_list_it ( fat_inode_cache 19 | fat_inode_cache 3 ← fat_cache 0 ( squashfs_inode_cache 188 | squashfs_inode_cache 87 ← jbd2_journal_head 1938 ( jbd2_revoke_table_s 25 ( ext4_inode_cache 469038 | ext4_inode_cache 7998 ← ext4_allocation_context ( ext4_io_end 1728 ( ext4_extent_status 244698 | ext4_extent_status 10710 mbcache 1898 ( fscrypt_info 1152 ( fscrypt_ctx 680 ( userfaultfd_ctx_cache ( dio 205 ( pid_namespace 312 ( posix_timers_cache 140 ( ip4-frags 23 ( xfrm_dst_cache 0 ( RAW 776 | RAW 896 tw_sock_TCP 68 ( request_sock_TCP 52 ( TCP 240 ( hugetlbfs_inode_cache ( dquot 256 ( eventpoll_pwq 6716 ( eventpoll_epi 8056 ( dax_cache 4 ( request_queue 40 ( dmaengine-unmap-256 ( dmaengine-unmap-128 ( sock_inode_cache 7046 ( file_lock_cache 321 ( net_namespace 33 ( shmem_inode_cache 7808 | shmem_inode_cache 7807 ← taskstats 879 ( proc_inode_cache 20991 | proc_inode_cache 7381 ← sigqueue 200 ( bdev_cache 105 ( kernfs_node_cache 146133 | kernfs_node_cache 141254 mnt_cache 4087 ( filp 43510 | filp 44230 inode_cache 34643 | inode_cache 23641 ← dentry 736367 | dentry 53630 ← iint_cache 0 ( lsm_file_cache 24003 | lsm_file_cache 24002 iint_cache 0 ( buffer_head 15177 | buffer_head 15417 nsproxy 1920 | nsproxy 1577 vm_area_struct 86939 | vm_area_struct 87250 mm_struct 1353 ( files_cache 1584 ( signal_cache 2357 ( sighand_cache 1434 ( task_struct 2364 | task_struct 2356 cred_jar 11118 | cred_jar 11706 anon_vma 41724 | anon_vma 41769 pid 74665 | pid 74853 Acpi-Operand 11627 ( Acpi-ParseExt 624 ( Acpi-State 1377 ( Acpi-Namespace 6936 ( numa_policy 31 ( trace_event_file 1970 ( ftrace_event_field 37941 | ftrace_event_field 17566 radix_tree_node 19962 | radix_tree_node 19578 task_group 420 ( dma-kmalloc-8192 0 ( dma-kmalloc-4096 0 ( dma-kmalloc-2048 0 ( dma-kmalloc-1024 0 ( dma-kmalloc-512 0 ( dma-kmalloc-256 0 ( dma-kmalloc-128 0 ( dma-kmalloc-64 0 ( dma-kmalloc-32 0 ( dma-kmalloc-16 0 ( dma-kmalloc-8 0 ( dma-kmalloc-192 0 ( dma-kmalloc-96 0 ( kmalloc-8192 607 | kmalloc-8192 588 kmalloc-4096 2803 | kmalloc-4096 2768 kmalloc-2048 5914 | kmalloc-2048 5749 kmalloc-1024 12056 | kmalloc-1024 12030 kmalloc-512 13772 | kmalloc-512 13765 kmalloc-256 10834 | kmalloc-256 10775 kmalloc-192 11681 | kmalloc-192 11147 kmalloc-128 10545 | kmalloc-128 7556 kmalloc-96 24952 | kmalloc-96 16292 kmalloc-64 144963 | kmalloc-64 43284 kmalloc-32 170695 | kmalloc-32 166264 kmalloc-16 32519 ( kmalloc-8 22539 ( kmem_cache_node 4544 ( kmem_cache 4347 | kmem_cache 3654
Free page cache and reclaimable slab objects
Use sysctl utility to issue command to the kernel to free page cache and reclaimable slab objects including dentries and inodes.
$ sudo sysctl -w vm.drop_caches=3
Disable logging messages
Issuing drop cache command will emit KERN_INFO level log message.
$ sudo sysctl -w vm.drop_caches=2
$ dmesg [...] [Wed Sep 5 03:02:39 2018] sysctl (12414): drop_caches: 2 [...]
$ echo 1 | sudo tee /proc/sys/vm/drop_caches
$ dmesg [...] [Wed Sep 5 03:05:38 2018] tee (12696): drop_caches: 1 [...]
This log message can be disabled, although this action will be logged.
$ sudo sysctl -w vm.drop_caches=4
$ dmesg [...] [Wed Sep 5 03:08:39 2018] sysctl (12730): drop_caches: 4 [...]
This will prevent further log messages, but it cannot be switched back on again without system reboot, after which it will resume standard logging operations.
