1LIBHUGETLBFS(7) Miscellaneous Information Manual LIBHUGETLBFS(7)
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6 libhugetlbfs - preload library to back text, data, malloc() or shared
7 memory with hugepages
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10 export [environment options]
11 [LD_PRELOAD=libhugetlbfs.so] target_application
12
14 libhugetlbfs is a library that can back application text, data, mal‐
15 loc() and shared memory with hugepages. This is of benefit to applica‐
16 tions that use large amounts of address space and suffer a performance
17 hit due to TLB misses. Wall-clock time or oprofile can be used to
18 determine if there is a performance benefit from using libhugetlbfs or
19 not. In all cases but shared memory, a hugetlbfs mount must exist and
20 a hugepage pool defined for hugepages to be used.
21 Some limited functionality is available for unmodified dynamically
23 linked applications. By preloading the library, the library can back
24 malloc() and shared memory, and text and data segments can be partially
25 backed if they are large enough.
26 For the effective backing of text and data with huge pages, the appli‐
28 cation must be linked to the library and the ELF segments correctly
29 aligned using the ld helpers. Once linked, malloc or shared memory can
30 still be backed but no pre-loading is required. See /usr/share/doc/lib‐
31 hugetlbfs/HOWTO and ld.hugetlbfs(1) for detailed instructions on
32 relinking applications.
33 For applications that are hugepage-aware and linked to the library
35 get_huge_pages() can be used for the direct allocation of hugepage-
36 backed regions.
37 Unless otherwise specified, libhugetlbfs will use the default hugepage
39 size to back memory regions. The default size is the value of Hugepage‐
40 size displayed in /proc/meminfo. The size can be specified in bytes or
41 in kilobytes, megabytes, or gigabytes by appending K, M, or G respec‐
42 tively. It is an error to specify a invalid, unsupported, or otherwise
43 unconfigured huge page size. Kernel 2.6.27 or later is required to
44 specify any pagesize other than the default.
45 See /usr/share/docs/libhugetlbfs/HOWTO for detailed instructions on how
47 the library should be used, particularly when relinking the applica‐
48 tion. This manual page provides a brief synopsis of the environment
49 variables as a quick reference.
50 The following variables affect what memory regions are backed by
52 hugepages. In all cases, the environment being unset implies the fea‐
53 ture should remain disabled.
54 HUGETLB_DEFAULT_PAGE_SIZE=<pagesize>
57 This sets the default hugepage size to be used by libhugetlbfs.
58 If not set, libhugetlbfs will use the kernel's default hugepage
59 size.
60 HUGETLB_MORECORE=[yes|<pagesize>]
63 This enables the hugepage malloc() feature, instructing lib‐
64 hugetlbfs to override glibc's normal morecore() function with a
65 hugepage version and use it for malloc(). All application mal‐
66 loc() memory should come from hugepage memory until it runs out,
67 it will then fallback to base pages. Note that applications
68 that use custom allocators may not be able to back their heaps
69 using hugepages and this environment variable. It may be neces‐
70 sary to modify the custom allocator to use get_huge_pages().
71 HUGETLB_SHM=yes
74 When this environment variable is set, the SHM_HUGETLB flag is
75 added to the shmget() call and the size parameter is aligned to
76 back the shared memory segment with hugepages. In the event
77 hugepages cannot be used, base pages will be used instead and a
78 warning will be printed to explain the failure. The pagesize
79 cannot be specified with this parameter. To change the kernels
80 default hugepage size, use the pagesize= kernel boot parameter
81 (2.6.26 or later required).
82 HUGETLB_ELFMAP=[no|[R[<=pagesize>]:[W[<=pagesize>]]]
85 If the application has been relinked (see the HOWTO for instruc‐
86 tions), this environment variable determines whether read-only,
87 read-write, both or no segments are backed by hugepages and what
88 pagesize should be used. If the recommended relinking method has
89 been used, then hugeedit can be used to automatically back the
90 text or data by default.
91 HUGETLB_FORCE_ELFMAP=yes
94 Force the use of hugepages for text and data segments even if
95 the application has not been relinked to align the ELF segments
96 on a hugepage boundary. Partial segment remapping is not guar‐
97 anteed to work and the segments must be large enough to contain
98 at least one hugepage for the remapping to occur.
99 The following options affect how libhugetlbfs behaves.
102 HUGETLB_RESTRICT_EXE=e1:e2:...:eN
105 By default, libhugetlbfs will act on any program that it is
106 loaded with, either via LD_PRELOAD or by explicitly linking with
107 -lhugetlbfs.
108 There are situations in which it is desirable to restrict lib‐
110 hugetlbfs' actions to specific programs. For example, some ISV
111 applications are wrapped in a series of scripts that invoke
112 bash, python, and/or perl. It is more convenient to set the
113 environment variables related to libhugetlbfs before invoking
114 the wrapper scripts, yet this has the unintended and undesirable
115 consequence of causing the script interpreters to use and con‐
116 sume hugepages. There is no obvious benefit to causing the
117 script interpreters to use hugepages, and there is a clear dis‐
118 advantage: fewer hugepages are available to the actual applica‐
119 tion.
120 To address this scenario, set HUGETLB_RESTRICT_EXE to a colon-
122 separated list of programs to which the other libhugetlbfs envi‐
123 ronment variables should apply. (If not set, libhugetlbfs will
124 attempt to apply the requested actions to all programs.) For
125 example,
126 HUGETLB_RESTRICT_EXE=hpcc:long_hpcc
128 will restrict libhugetlbfs' actions to programs named
130 /home/fred/hpcc and /bench/long_hpcc but not /bin/hpcc_no.
131 HUGETLB_MORECORE_SHRINK=yes
135 By default, the hugepage heap does not shrink. Shrinking is
136 enabled by setting this environment variable. It is disabled by
137 default as glibc occasionally exhibits strange behaviour if it
138 mistakes the heap returned by libhugetlbfs as a foreign brk().
139 HUGETLB_NO_PREFAULT
142 By default libhugetlbfs will prefault regions it creates to
143 ensure they can be referenced without receiving a SIGKILL. On
144 kernels older than 2.6.27, this was necessary as the system did
145 not guarantee that future faults would succeed on regions mapped
146 MAP_PRIVATE. Prefaulting impacts the performance of malloc()
147 and can result in poor placement on NUMA systems. If it is known
148 the hugepage pool is large enough to run the application or the
149 kernel is 2.6.27 or later, this environment variable should be
150 set.
151 HUGETLB_NO_RESERVE=yes
154 By default, the kernel will reserve huge pages at mmap() time to
156 ensure that future faults will succeed. This avoids unexpected
157 application failure at fault time but some applications depend
158 on memory overcommit to create large sparse mappings. For this
159 type of application, setting this environment variable will cre‐
160 ate huge page backed mappings without a reservation. Use this
161 option with extreme care as in the event huge pages are not
162 available when the mapping is used, the application will be
163 killed. On older kernels, the use of this feature can trigger
164 the OOM killer. Hence, even with this variable set, reservations
165 may still be used for safety.
166 HUGETLB_MORECORE_HEAPBASE=address
169 libhugetlbfs normally picks an address to use as the base of the
170 heap for malloc() automatically. This environment variable fixes
171 which address is used.
172 HUGETLB_PATH=<path>
175 The path to the hugetlbfs mount is automatically determined at
176 run-time. In the event there are multiple mounts and the wrong
177 one is being selected, use this option to select the correct
178 one. This may be the case if an application-specific mount with
179 a fixed quota has been created for example.
180 HUGETLB_SHARE=1
183 By default, libhugetlbfs uses unlinked hugetlbfs files to store
184 remapped program segment data. If the same program is started
185 multiple times using hugepage segments, multiple hugepages will
186 be used to store the same program data. The reduce this wastage,
187 setting this environment variable will share read-only segments
188 between multiple invocations of a program at the cost of the
189 memory being used whether the applications are running or not.
190 It is also possible that a malicious application inferfere with
191 other applications executable code. See the HOWTO for more
192 detailed information on this topic.
193 The following options control the verbosity of libhugetlbfs.
196 HUGETLB_VERBOSE=<level>
199 The default value for this is 1 and the range of the value is
200 from 0 to 99. The higher the value, the more verbose the output
201 is. 0 is quiet and 3 will output much debugging information.
202 HUGETLB_DEBUG
205 Once set, this will give very detailed output on what is happen‐
206 ing in the library and run extra diagnostics.
207
210 [DESTDIR|/usr/share]/doc/libhugetlbfs/HOWTO
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214 oprofile(1), ld.hugetlbfs(1), hugectl(8), hugeedit(8), gethugepage‐
215 size(3), gethugepagesizes(3), getpagesizes(3), hugetlbfs_test_path(3),
216 hugetlbfs_find_path(3), hugetlbfs_find_path_for_size(3),
217 hugetlbfs_test_path(3), hugetlbfs_test_path_for_size(3),
218 hugetlbfs_unlinked_fd(3), hugetlbfs_unlinked_fd_for_size(3),
219 get_huge_pages(3), free_huge_pages(3)
220
222 libhugetlbfs was written by various people on the libhugetlbfs-devel
223 mailing list.
224 September 27, 2008 LIBHUGETLBFS(7)