diff options
Diffstat (limited to 'Documentation/vm')
| -rw-r--r-- | Documentation/vm/00-INDEX | 2 | ||||
| -rw-r--r-- | Documentation/vm/idle_page_tracking.txt | 98 | ||||
| -rw-r--r-- | Documentation/vm/pagemap.txt | 13 | ||||
| -rw-r--r-- | Documentation/vm/zswap.txt | 36 |
4 files changed, 140 insertions, 9 deletions
diff --git a/Documentation/vm/00-INDEX b/Documentation/vm/00-INDEX index 081c49777abb..6a5e2a102a45 100644 --- a/Documentation/vm/00-INDEX +++ b/Documentation/vm/00-INDEX @@ -14,6 +14,8 @@ hugetlbpage.txt - a brief summary of hugetlbpage support in the Linux kernel. hwpoison.txt - explains what hwpoison is +idle_page_tracking.txt + - description of the idle page tracking feature. ksm.txt - how to use the Kernel Samepage Merging feature. numa diff --git a/Documentation/vm/idle_page_tracking.txt b/Documentation/vm/idle_page_tracking.txt new file mode 100644 index 000000000000..85dcc3bb85dc --- /dev/null +++ b/Documentation/vm/idle_page_tracking.txt @@ -0,0 +1,98 @@ +MOTIVATION + +The idle page tracking feature allows to track which memory pages are being +accessed by a workload and which are idle. This information can be useful for +estimating the workload's working set size, which, in turn, can be taken into +account when configuring the workload parameters, setting memory cgroup limits, +or deciding where to place the workload within a compute cluster. + +It is enabled by CONFIG_IDLE_PAGE_TRACKING=y. + +USER API + +The idle page tracking API is located at /sys/kernel/mm/page_idle. Currently, +it consists of the only read-write file, /sys/kernel/mm/page_idle/bitmap. + +The file implements a bitmap where each bit corresponds to a memory page. The +bitmap is represented by an array of 8-byte integers, and the page at PFN #i is +mapped to bit #i%64 of array element #i/64, byte order is native. When a bit is +set, the corresponding page is idle. + +A page is considered idle if it has not been accessed since it was marked idle +(for more details on what "accessed" actually means see the IMPLEMENTATION +DETAILS section). To mark a page idle one has to set the bit corresponding to +the page by writing to the file. A value written to the file is OR-ed with the +current bitmap value. + +Only accesses to user memory pages are tracked. These are pages mapped to a +process address space, page cache and buffer pages, swap cache pages. For other +page types (e.g. SLAB pages) an attempt to mark a page idle is silently ignored, +and hence such pages are never reported idle. + +For huge pages the idle flag is set only on the head page, so one has to read +/proc/kpageflags in order to correctly count idle huge pages. + +Reading from or writing to /sys/kernel/mm/page_idle/bitmap will return +-EINVAL if you are not starting the read/write on an 8-byte boundary, or +if the size of the read/write is not a multiple of 8 bytes. Writing to +this file beyond max PFN will return -ENXIO. + +That said, in order to estimate the amount of pages that are not used by a +workload one should: + + 1. Mark all the workload's pages as idle by setting corresponding bits in + /sys/kernel/mm/page_idle/bitmap. The pages can be found by reading + /proc/pid/pagemap if the workload is represented by a process, or by + filtering out alien pages using /proc/kpagecgroup in case the workload is + placed in a memory cgroup. + + 2. Wait until the workload accesses its working set. + + 3. Read /sys/kernel/mm/page_idle/bitmap and count the number of bits set. If + one wants to ignore certain types of pages, e.g. mlocked pages since they + are not reclaimable, he or she can filter them out using /proc/kpageflags. + +See Documentation/vm/pagemap.txt for more information about /proc/pid/pagemap, +/proc/kpageflags, and /proc/kpagecgroup. + +IMPLEMENTATION DETAILS + +The kernel internally keeps track of accesses to user memory pages in order to +reclaim unreferenced pages first on memory shortage conditions. A page is +considered referenced if it has been recently accessed via a process address +space, in which case one or more PTEs it is mapped to will have the Accessed bit +set, or marked accessed explicitly by the kernel (see mark_page_accessed()). The +latter happens when: + + - a userspace process reads or writes a page using a system call (e.g. read(2) + or write(2)) + + - a page that is used for storing filesystem buffers is read or written, + because a process needs filesystem metadata stored in it (e.g. lists a + directory tree) + + - a page is accessed by a device driver using get_user_pages() + +When a dirty page is written to swap or disk as a result of memory reclaim or +exceeding the dirty memory limit, it is not marked referenced. + +The idle memory tracking feature adds a new page flag, the Idle flag. This flag +is set manually, by writing to /sys/kernel/mm/page_idle/bitmap (see the USER API +section), and cleared automatically whenever a page is referenced as defined +above. + +When a page is marked idle, the Accessed bit must be cleared in all PTEs it is +mapped to, otherwise we will not be able to detect accesses to the page coming +from a process address space. To avoid interference with the reclaimer, which, +as noted above, uses the Accessed bit to promote actively referenced pages, one +more page flag is introduced, the Young flag. When the PTE Accessed bit is +cleared as a result of setting or updating a page's Idle flag, the Young flag +is set on the page. The reclaimer treats the Young flag as an extra PTE +Accessed bit and therefore will consider such a page as referenced. + +Since the idle memory tracking feature is based on the memory reclaimer logic, +it only works with pages that are on an LRU list, other pages are silently +ignored. That means it will ignore a user memory page if it is isolated, but +since there are usually not many of them, it should not affect the overall +result noticeably. In order not to stall scanning of the idle page bitmap, +locked pages may be skipped too. diff --git a/Documentation/vm/pagemap.txt b/Documentation/vm/pagemap.txt index 3cd38438242a..0e1e55588b59 100644 --- a/Documentation/vm/pagemap.txt +++ b/Documentation/vm/pagemap.txt @@ -5,7 +5,7 @@ pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow userspace programs to examine the page tables and related information by reading files in /proc. -There are three components to pagemap: +There are four components to pagemap: * /proc/pid/pagemap. This file lets a userspace process find out which physical frame each virtual page is mapped to. It contains one 64-bit @@ -70,6 +70,11 @@ There are three components to pagemap: 22. THP 23. BALLOON 24. ZERO_PAGE + 25. IDLE + + * /proc/kpagecgroup. This file contains a 64-bit inode number of the + memory cgroup each page is charged to, indexed by PFN. Only available when + CONFIG_MEMCG is set. Short descriptions to the page flags: @@ -116,6 +121,12 @@ Short descriptions to the page flags: 24. ZERO_PAGE zero page for pfn_zero or huge_zero page +25. IDLE + page has not been accessed since it was marked idle (see + Documentation/vm/idle_page_tracking.txt). Note that this flag may be + stale in case the page was accessed via a PTE. To make sure the flag + is up-to-date one has to read /sys/kernel/mm/page_idle/bitmap first. + [IO related page flags] 1. ERROR IO error occurred 3. UPTODATE page has up-to-date data diff --git a/Documentation/vm/zswap.txt b/Documentation/vm/zswap.txt index 8458c0861e4e..89fff7d611cc 100644 --- a/Documentation/vm/zswap.txt +++ b/Documentation/vm/zswap.txt @@ -32,7 +32,7 @@ can also be enabled and disabled at runtime using the sysfs interface. An example command to enable zswap at runtime, assuming sysfs is mounted at /sys, is: -echo 1 > /sys/modules/zswap/parameters/enabled +echo 1 > /sys/module/zswap/parameters/enabled When zswap is disabled at runtime it will stop storing pages that are being swapped out. However, it will _not_ immediately write out or fault @@ -49,14 +49,26 @@ Zswap receives pages for compression through the Frontswap API and is able to evict pages from its own compressed pool on an LRU basis and write them back to the backing swap device in the case that the compressed pool is full. -Zswap makes use of zbud for the managing the compressed memory pool. Each -allocation in zbud is not directly accessible by address. Rather, a handle is +Zswap makes use of zpool for the managing the compressed memory pool. Each +allocation in zpool is not directly accessible by address. Rather, a handle is returned by the allocation routine and that handle must be mapped before being accessed. The compressed memory pool grows on demand and shrinks as compressed -pages are freed. The pool is not preallocated. +pages are freed. The pool is not preallocated. By default, a zpool of type +zbud is created, but it can be selected at boot time by setting the "zpool" +attribute, e.g. zswap.zpool=zbud. It can also be changed at runtime using the +sysfs "zpool" attribute, e.g. + +echo zbud > /sys/module/zswap/parameters/zpool + +The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which +means the compression ratio will always be 2:1 or worse (because of half-full +zbud pages). The zsmalloc type zpool has a more complex compressed page +storage method, and it can achieve greater storage densities. However, +zsmalloc does not implement compressed page eviction, so once zswap fills it +cannot evict the oldest page, it can only reject new pages. When a swap page is passed from frontswap to zswap, zswap maintains a mapping -of the swap entry, a combination of the swap type and swap offset, to the zbud +of the swap entry, a combination of the swap type and swap offset, to the zpool handle that references that compressed swap page. This mapping is achieved with a red-black tree per swap type. The swap offset is the search key for the tree nodes. @@ -74,9 +86,17 @@ controlled policy: * max_pool_percent - The maximum percentage of memory that the compressed pool can occupy. -Zswap allows the compressor to be selected at kernel boot time by setting the -“compressor” attribute. The default compressor is lzo. e.g. -zswap.compressor=deflate +The default compressor is lzo, but it can be selected at boot time by setting +the “compressor” attribute, e.g. zswap.compressor=lzo. It can also be changed +at runtime using the sysfs "compressor" attribute, e.g. + +echo lzo > /sys/module/zswap/parameters/compressor + +When the zpool and/or compressor parameter is changed at runtime, any existing +compressed pages are not modified; they are left in their own zpool. When a +request is made for a page in an old zpool, it is uncompressed using its +original compressor. Once all pages are removed from an old zpool, the zpool +and its compressor are freed. A debugfs interface is provided for various statistic about pool size, number of pages stored, and various counters for the reasons pages are rejected. |
