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-rw-r--r--Documentation/admin-guide/mm/transhuge.rst57
1 files changed, 34 insertions, 23 deletions
diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index bd5714547cee..8ee78ec232eb 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -191,7 +191,14 @@ allocation failure to throttle the next allocation attempt::
/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
-The khugepaged progress can be seen in the number of pages collapsed::
+The khugepaged progress can be seen in the number of pages collapsed (note
+that this counter may not be an exact count of the number of pages
+collapsed, since "collapsed" could mean multiple things: (1) A PTE mapping
+being replaced by a PMD mapping, or (2) All 4K physical pages replaced by
+one 2M hugepage. Each may happen independently, or together, depending on
+the type of memory and the failures that occur. As such, this value should
+be interpreted roughly as a sign of progress, and counters in /proc/vmstat
+consulted for more accurate accounting)::
/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
@@ -220,6 +227,13 @@ memory. A lower value can prevent THPs from being
collapsed, resulting fewer pages being collapsed into
THPs, and lower memory access performance.
+``max_ptes_shared`` specifies how many pages can be shared across multiple
+processes. Exceeding the number would block the collapse::
+
+ /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared
+
+A higher value may increase memory footprint for some workloads.
+
Boot parameter
==============
@@ -298,8 +312,7 @@ monitor how successfully the system is providing huge pages for use.
thp_fault_alloc
is incremented every time a huge page is successfully
- allocated to handle a page fault. This applies to both the
- first time a page is faulted and for COW faults.
+ allocated to handle a page fault.
thp_collapse_alloc
is incremented by khugepaged when it has found
@@ -310,6 +323,11 @@ thp_fault_fallback
is incremented if a page fault fails to allocate
a huge page and instead falls back to using small pages.
+thp_fault_fallback_charge
+ is incremented if a page fault fails to charge a huge page and
+ instead falls back to using small pages even though the
+ allocation was successful.
+
thp_collapse_alloc_failed
is incremented if khugepaged found a range
of pages that should be collapsed into one huge page but failed
@@ -319,6 +337,15 @@ thp_file_alloc
is incremented every time a file huge page is successfully
allocated.
+thp_file_fallback
+ is incremented if a file huge page is attempted to be allocated
+ but fails and instead falls back to using small pages.
+
+thp_file_fallback_charge
+ is incremented if a file huge page cannot be charged and instead
+ falls back to using small pages even though the allocation was
+ successful.
+
thp_file_mapped
is incremented every time a file huge page is mapped into
user address space.
@@ -346,10 +373,9 @@ thp_split_pmd
page table entry.
thp_zero_page_alloc
- is incremented every time a huge zero page is
- successfully allocated. It includes allocations which where
- dropped due race with other allocation. Note, it doesn't count
- every map of the huge zero page, only its allocation.
+ is incremented every time a huge zero page used for thp is
+ successfully allocated. Note, it doesn't count every map of
+ the huge zero page, only its allocation.
thp_zero_page_alloc_failed
is incremented if kernel fails to allocate
@@ -381,23 +407,8 @@ compact_fail
is incremented if the system tries to compact memory
but failed.
-compact_pages_moved
- is incremented each time a page is moved. If
- this value is increasing rapidly, it implies that the system
- is copying a lot of data to satisfy the huge page allocation.
- It is possible that the cost of copying exceeds any savings
- from reduced TLB misses.
-
-compact_pagemigrate_failed
- is incremented when the underlying mechanism
- for moving a page failed.
-
-compact_blocks_moved
- is incremented each time memory compaction examines
- a huge page aligned range of pages.
-
It is possible to establish how long the stalls were using the function
-tracer to record how long was spent in __alloc_pages_nodemask and
+tracer to record how long was spent in __alloc_pages() and
using the mm_page_alloc tracepoint to identify which allocations were
for huge pages.