On 2024/4/24 22:20, Ryan Roberts wrote:
On 24/04/2024 14:49, Baolin Wang wrote:
On 2024/4/24 18:01, Ryan Roberts wrote:
On 24/04/2024 10:55, Baolin Wang wrote:
On 2024/4/24 16:26, Ryan Roberts wrote:
On 24/04/2024 07:55, Baolin Wang wrote:
On 2024/4/23 18:41, Ryan Roberts wrote:
On 22/04/2024 08:02, Baolin Wang wrote:
Anonymous pages have already been supported for multi-size (mTHP) allocation
through commit 19eaf44954df, that can allow THP to be configured through the
sysfs interface located at
'/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.
However, the anonymous shared pages will ignore the anonymous mTHP rule
configured through the sysfs interface, and can only use the PMD-mapped
THP, that is not reasonable. Many implement anonymous page sharing through
mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios,
therefore, users expect to apply an unified mTHP strategy for anonymous
pages,
also including the anonymous shared pages, in order to enjoy the benefits of
mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat
than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss
etc.
This sounds like a very useful addition!
Out of interest, can you point me at any workloads (and off-the-shelf
benchmarks
for those workloads) that predominantly use shared anon memory?
As far as I know, some database related workloads make extensive use of shared
anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also
uses shared anonymous memory. And I still need to do some investigation to
measure the performance.
[1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL
Thanks for the pointer!
The primary strategy is that, the use of huge pages for anonymous shared
pages
still follows the global control determined by the mount option "huge="
parameter
or the sysfs interface at
'/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
The utilization of mTHP is allowed only when the global 'huge' switch is
enabled.
Subsequently, the mTHP sysfs interface
(/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled)
is checked to determine the mTHP size that can be used for large folio
allocation
for these anonymous shared pages.
I'm not sure about this proposed control mechanism; won't it break
compatibility? I could be wrong, but I don't think shmem's use of THP used to
depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it
Yes, I realized this after more testing.
doesn't make sense to me that we now depend upon the
/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by
default disables all sizes except 2M, which is set to "inherit" from
/sys/kernel/mm/transparent_hugepage/enabled).
The other problem is that shmem_enabled has a different set of options
(always/never/within_size/advise/deny/force) to enabled
(always/madvise/never)
Perhaps it would be cleaner to do the same trick we did for enabled;
Introduce
/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the
same values as the top-level
/sys/kernel/mm/transparent_hugepage/shmem_enabled,
plus the additional "inherit" option. By default all sizes will be set to
"never" except 2M, which is set to "inherit".
Sounds good to me. But I do not want to copy all same values from top-level
'/sys/kernel/mm/transparent_hugepage/shmem_enabled':
always within_size advise never deny force
For mTHP's shmem_enabled interface, we can just keep below values:
always within_size advise never
Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to
'never', and 'force' is equal to 'always'.
I'll admit it wasn't completely clear to me after reading the docs, but my
rough
understanding is:
- /sys/kernel/mm/transparent_hugepage/shmem_enabled controls
mmap(SHARED|ANON) allocations (mostly; see rule 3)
- huge=... controls tmpfs allocations
- deny and force in shmem_enabled are equivalent to never and always for
mmap(SHARED|ANON) but additionally override all tmpfs mounts so they
act as
if they were mounted with huge=never or huge=always
Is that correct? If so, then I think it still makes sense to support per-size
Correct.
deny/force. Certainly if a per-size control is set to "inherit" and the
top-level control is set to deny or force, you would need that to mean
something.
IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface
should only control the anonymous shmem. And 'huge=' controls tmpfs allocation,
so we should not use anonymous control to override tmpfs control, which seems a
little mess?
I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave the
tmpfs stuff for another time. But my point is that
/mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the
value is deny or force. So if you have:
echo deny > /mm/transparent_hugepage/shmem_enabled
IIUC, this global control will cause shmem_is_huge() to always return false, so
no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set,
anonymous shmem will not use mTHP. No?
No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and
I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow
the established pattern.
For anon-private, each size is controlled by its
/mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is
"inherit", in which case the value in /mm/transparent_hugepage/enabled is used
for that size.
That approach enables us to 1) maintain back-compat and 2) control each size
independently
1) is met because the default is that all sizes are initially set to "never",
except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled)
which is initially set to inherit. So any mTHP unaware SW can still modify
/mm/transparent_hugepage/enabled and it will still only apply to PMD size.
2) is met because mTHP aware SW can come along and e.g. enable the 64K size
(echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without having to
modify the value in /mm/transparent_hugepage/enabled.
Thanks for explanation. Initially, I want to make
‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge
page, but I think it should follow the same strategy as anon mTHP as you
said.
echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled
What does that mean?
So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to
support the deny and force values. When applied to non-PMD sizes, "deny" can
just be a noop for now, because there was no way to configure a tmpfs mount for
non-PMD size THP in the first place. But I'm not sure what to do with "force"?
OK. And I also prefer that "force" should be a noop too, since anon
shmem control should not configure tmpfs huge page allocation.
