diff options
Diffstat (limited to 'Documentation/vm')
| -rw-r--r-- | Documentation/vm/balance | 14 | ||||
| -rw-r--r-- | Documentation/vm/page_migration | 27 | ||||
| -rw-r--r-- | Documentation/vm/slub.txt | 59 | ||||
| -rw-r--r-- | Documentation/vm/split_page_table_lock | 4 | ||||
| -rw-r--r-- | Documentation/vm/transhuge.txt | 10 | ||||
| -rw-r--r-- | Documentation/vm/unevictable-lru.txt | 120 |
6 files changed, 110 insertions, 124 deletions
diff --git a/Documentation/vm/balance b/Documentation/vm/balance index c46e68cf9..964595481 100644 --- a/Documentation/vm/balance +++ b/Documentation/vm/balance @@ -1,12 +1,14 @@ Started Jan 2000 by Kanoj Sarcar <kanoj@sgi.com> -Memory balancing is needed for non __GFP_WAIT as well as for non -__GFP_IO allocations. +Memory balancing is needed for !__GFP_ATOMIC and !__GFP_KSWAPD_RECLAIM as +well as for non __GFP_IO allocations. -There are two reasons to be requesting non __GFP_WAIT allocations: -the caller can not sleep (typically intr context), or does not want -to incur cost overheads of page stealing and possible swap io for -whatever reasons. +The first reason why a caller may avoid reclaim is that the caller can not +sleep due to holding a spinlock or is in interrupt context. The second may +be that the caller is willing to fail the allocation without incurring the +overhead of page reclaim. This may happen for opportunistic high-order +allocation requests that have order-0 fallback options. In such cases, +the caller may also wish to avoid waking kswapd. __GFP_IO allocation requests are made to prevent file system deadlocks. diff --git a/Documentation/vm/page_migration b/Documentation/vm/page_migration index 6513fe2d9..fea5c0864 100644 --- a/Documentation/vm/page_migration +++ b/Documentation/vm/page_migration @@ -92,29 +92,26 @@ Steps: 2. Insure that writeback is complete. -3. Prep the new page that we want to move to. It is locked - and set to not being uptodate so that all accesses to the new - page immediately lock while the move is in progress. +3. Lock the new page that we want to move to. It is locked so that accesses to + this (not yet uptodate) page immediately lock while the move is in progress. -4. The new page is prepped with some settings from the old page so that - accesses to the new page will discover a page with the correct settings. - -5. All the page table references to the page are converted - to migration entries or dropped (nonlinear vmas). - This decrease the mapcount of a page. If the resulting - mapcount is not zero then we do not migrate the page. - All user space processes that attempt to access the page - will now wait on the page lock. +4. All the page table references to the page are converted to migration + entries. This decreases the mapcount of a page. If the resulting + mapcount is not zero then we do not migrate the page. All user space + processes that attempt to access the page will now wait on the page lock. -6. The radix tree lock is taken. This will cause all processes trying +5. The radix tree lock is taken. This will cause all processes trying to access the page via the mapping to block on the radix tree spinlock. -7. The refcount of the page is examined and we back out if references remain +6. The refcount of the page is examined and we back out if references remain otherwise we know that we are the only one referencing this page. -8. The radix tree is checked and if it does not contain the pointer to this +7. The radix tree is checked and if it does not contain the pointer to this page then we back out because someone else modified the radix tree. +8. The new page is prepped with some settings from the old page so that + accesses to the new page will discover a page with the correct settings. + 9. The radix tree is changed to point to the new page. 10. The reference count of the old page is dropped because the radix tree diff --git a/Documentation/vm/slub.txt b/Documentation/vm/slub.txt index b0c6d1bbb..699d8ea5c 100644 --- a/Documentation/vm/slub.txt +++ b/Documentation/vm/slub.txt @@ -280,4 +280,63 @@ of other objects. slub_debug=FZ,dentry +Extended slabinfo mode and plotting +----------------------------------- + +The slabinfo tool has a special 'extended' ('-X') mode that includes: + - Slabcache Totals + - Slabs sorted by size (up to -N <num> slabs, default 1) + - Slabs sorted by loss (up to -N <num> slabs, default 1) + +Additionally, in this mode slabinfo does not dynamically scale sizes (G/M/K) +and reports everything in bytes (this functionality is also available to +other slabinfo modes via '-B' option) which makes reporting more precise and +accurate. Moreover, in some sense the `-X' mode also simplifies the analysis +of slabs' behaviour, because its output can be plotted using the +slabinfo-gnuplot.sh script. So it pushes the analysis from looking through +the numbers (tons of numbers) to something easier -- visual analysis. + +To generate plots: +a) collect slabinfo extended records, for example: + + while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done + +b) pass stats file(-s) to slabinfo-gnuplot.sh script: + slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN] + +The slabinfo-gnuplot.sh script will pre-processes the collected records +and generates 3 png files (and 3 pre-processing cache files) per STATS +file: + - Slabcache Totals: FOO_STATS-totals.png + - Slabs sorted by size: FOO_STATS-slabs-by-size.png + - Slabs sorted by loss: FOO_STATS-slabs-by-loss.png + +Another use case, when slabinfo-gnuplot can be useful, is when you need +to compare slabs' behaviour "prior to" and "after" some code modification. +To help you out there, slabinfo-gnuplot.sh script can 'merge' the +`Slabcache Totals` sections from different measurements. To visually +compare N plots: + +a) Collect as many STATS1, STATS2, .. STATSN files as you need + while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done + +b) Pre-process those STATS files + slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN + +c) Execute slabinfo-gnuplot.sh in '-t' mode, passing all of the +generated pre-processed *-totals + slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals + +This will produce a single plot (png file). + +Plots, expectedly, can be large so some fluctuations or small spikes +can go unnoticed. To deal with that, `slabinfo-gnuplot.sh' has two +options to 'zoom-in'/'zoom-out': + a) -s %d,%d overwrites the default image width and heigh + b) -r %d,%d specifies a range of samples to use (for example, + in `slabinfo -X >> FOO_STATS; sleep 1;' case, using + a "-r 40,60" range will plot only samples collected + between 40th and 60th seconds). + Christoph Lameter, May 30, 2007 +Sergey Senozhatsky, October 23, 2015 diff --git a/Documentation/vm/split_page_table_lock b/Documentation/vm/split_page_table_lock index 6dea4fd5c..62842a857 100644 --- a/Documentation/vm/split_page_table_lock +++ b/Documentation/vm/split_page_table_lock @@ -54,8 +54,8 @@ everything required is done by pgtable_page_ctor() and pgtable_page_dtor(), which must be called on PTE table allocation / freeing. Make sure the architecture doesn't use slab allocator for page table -allocation: slab uses page->slab_cache and page->first_page for its pages. -These fields share storage with page->ptl. +allocation: slab uses page->slab_cache for its pages. +This field shares storage with page->ptl. PMD split lock only makes sense if you have more than two page table levels. diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt index 8143b9e83..8a282687e 100644 --- a/Documentation/vm/transhuge.txt +++ b/Documentation/vm/transhuge.txt @@ -170,6 +170,16 @@ A lower value leads to gain less thp performance. Value of max_ptes_none can waste cpu time very little, you can ignore it. +max_ptes_swap specifies how many pages can be brought in from +swap when collapsing a group of pages into a transparent huge page. + +/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap + +A higher value can cause excessive swap IO and waste +memory. A lower value can prevent THPs from being +collapsed, resulting fewer pages being collapsed into +THPs, and lower memory access performance. + == Boot parameter == You can change the sysfs boot time defaults of Transparent Hugepage diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt index 32ee3a67d..fa3b52708 100644 --- a/Documentation/vm/unevictable-lru.txt +++ b/Documentation/vm/unevictable-lru.txt @@ -531,83 +531,20 @@ map. try_to_unmap() is always called, by either vmscan for reclaim or for page migration, with the argument page locked and isolated from the LRU. Separate -functions handle anonymous and mapped file pages, as these types of pages have -different reverse map mechanisms. - - (*) try_to_unmap_anon() - - To unmap anonymous pages, each VMA in the list anchored in the anon_vma - must be visited - at least until a VM_LOCKED VMA is encountered. If the - page is being unmapped for migration, VM_LOCKED VMAs do not stop the - process because mlocked pages are migratable. However, for reclaim, if - the page is mapped into a VM_LOCKED VMA, the scan stops. - - try_to_unmap_anon() attempts to acquire in read mode the mmap semaphore of - the mm_struct to which the VMA belongs. If this is successful, it will - mlock the page via mlock_vma_page() - we wouldn't have gotten to - try_to_unmap_anon() if the page were already mlocked - and will return - SWAP_MLOCK, indicating that the page is unevictable. - - If the mmap semaphore cannot be acquired, we are not sure whether the page - is really unevictable or not. In this case, try_to_unmap_anon() will - return SWAP_AGAIN. - - (*) try_to_unmap_file() - linear mappings - - Unmapping of a mapped file page works the same as for anonymous mappings, - except that the scan visits all VMAs that map the page's index/page offset - in the page's mapping's reverse map priority search tree. It also visits - each VMA in the page's mapping's non-linear list, if the list is - non-empty. - - As for anonymous pages, on encountering a VM_LOCKED VMA for a mapped file - page, try_to_unmap_file() will attempt to acquire the associated - mm_struct's mmap semaphore to mlock the page, returning SWAP_MLOCK if this - is successful, and SWAP_AGAIN, if not. - - (*) try_to_unmap_file() - non-linear mappings - - If a page's mapping contains a non-empty non-linear mapping VMA list, then - try_to_un{map|lock}() must also visit each VMA in that list to determine - whether the page is mapped in a VM_LOCKED VMA. Again, the scan must visit - all VMAs in the non-linear list to ensure that the pages is not/should not - be mlocked. - - If a VM_LOCKED VMA is found in the list, the scan could terminate. - However, there is no easy way to determine whether the page is actually - mapped in a given VMA - either for unmapping or testing whether the - VM_LOCKED VMA actually pins the page. - - try_to_unmap_file() handles non-linear mappings by scanning a certain - number of pages - a "cluster" - in each non-linear VMA associated with the - page's mapping, for each file mapped page that vmscan tries to unmap. If - this happens to unmap the page we're trying to unmap, try_to_unmap() will - notice this on return (page_mapcount(page) will be 0) and return - SWAP_SUCCESS. Otherwise, it will return SWAP_AGAIN, causing vmscan to - recirculate this page. We take advantage of the cluster scan in - try_to_unmap_cluster() as follows: - - For each non-linear VMA, try_to_unmap_cluster() attempts to acquire the - mmap semaphore of the associated mm_struct for read without blocking. - - If this attempt is successful and the VMA is VM_LOCKED, - try_to_unmap_cluster() will retain the mmap semaphore for the scan; - otherwise it drops it here. - - Then, for each page in the cluster, if we're holding the mmap semaphore - for a locked VMA, try_to_unmap_cluster() calls mlock_vma_page() to - mlock the page. This call is a no-op if the page is already locked, - but will mlock any pages in the non-linear mapping that happen to be - unlocked. - - If one of the pages so mlocked is the page passed in to try_to_unmap(), - try_to_unmap_cluster() will return SWAP_MLOCK, rather than the default - SWAP_AGAIN. This will allow vmscan to cull the page, rather than - recirculating it on the inactive list. - - Again, if try_to_unmap_cluster() cannot acquire the VMA's mmap sem, it - returns SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED - VMA, but couldn't be mlocked. +functions handle anonymous and mapped file and KSM pages, as these types of +pages have different reverse map lookup mechanisms, with different locking. +In each case, whether rmap_walk_anon() or rmap_walk_file() or rmap_walk_ksm(), +it will call try_to_unmap_one() for every VMA which might contain the page. + +When trying to reclaim, if try_to_unmap_one() finds the page in a VM_LOCKED +VMA, it will then mlock the page via mlock_vma_page() instead of unmapping it, +and return SWAP_MLOCK to indicate that the page is unevictable: and the scan +stops there. + +mlock_vma_page() is called while holding the page table's lock (in addition +to the page lock, and the rmap lock): to serialize against concurrent mlock or +munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim, +holepunching, and truncation of file pages and their anonymous COWed pages. try_to_munlock() REVERSE MAP SCAN @@ -623,29 +560,15 @@ all PTEs from the page. For this purpose, the unevictable/mlock infrastructure introduced a variant of try_to_unmap() called try_to_munlock(). try_to_munlock() calls the same functions as try_to_unmap() for anonymous and -mapped file pages with an additional argument specifying unlock versus unmap +mapped file and KSM pages with a flag argument specifying unlock versus unmap processing. Again, these functions walk the respective reverse maps looking -for VM_LOCKED VMAs. When such a VMA is found for anonymous pages and file -pages mapped in linear VMAs, as in the try_to_unmap() case, the functions -attempt to acquire the associated mmap semaphore, mlock the page via -mlock_vma_page() and return SWAP_MLOCK. This effectively undoes the -pre-clearing of the page's PG_mlocked done by munlock_vma_page. - -If try_to_unmap() is unable to acquire a VM_LOCKED VMA's associated mmap -semaphore, it will return SWAP_AGAIN. This will allow shrink_page_list() to -recycle the page on the inactive list and hope that it has better luck with the -page next time. - -For file pages mapped into non-linear VMAs, the try_to_munlock() logic works -slightly differently. On encountering a VM_LOCKED non-linear VMA that might -map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking the -page. munlock_vma_page() will just leave the page unlocked and let vmscan deal -with it - the usual fallback position. +for VM_LOCKED VMAs. When such a VMA is found, as in the try_to_unmap() case, +the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK. This +undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page. Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA. -However, the scan can terminate when it encounters a VM_LOCKED VMA and can -successfully acquire the VMA's mmap semaphore for read and mlock the page. +However, the scan can terminate when it encounters a VM_LOCKED VMA. Although try_to_munlock() might be called a great many times when munlocking a large region or tearing down a large address space that has been mlocked via mlockall(), overall this is a fairly rare event. @@ -673,11 +596,6 @@ Some examples of these unevictable pages on the LRU lists are: (3) mlocked pages that could not be isolated from the LRU and moved to the unevictable list in mlock_vma_page(). - (4) Pages mapped into multiple VM_LOCKED VMAs, but try_to_munlock() couldn't - acquire the VMA's mmap semaphore to test the flags and set PageMlocked. - munlock_vma_page() was forced to let the page back on to the normal LRU - list for vmscan to handle. - shrink_inactive_list() also diverts any unevictable pages that it finds on the inactive lists to the appropriate zone's unevictable list. |