On 2022/4/30 9:19 PM, Bagas Sanjaya wrote:
Hi Qi,
On Fri, Apr 29, 2022 at 09:35:52PM +0800, Qi Zheng wrote:
+Now in order to pursue high performance, applications mostly use some
+high-performance user-mode memory allocators, such as jemalloc or tcmalloc.
+These memory allocators use madvise(MADV_DONTNEED or MADV_FREE) to release
+physical memory for the following reasons::
+
+ First of all, we should hold as few write locks of mmap_lock as possible,
+ since the mmap_lock semaphore has long been a contention point in the
+ memory management subsystem. The mmap()/munmap() hold the write lock, and
+ the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
+ madvise() instead of munmap() to released physical memory can reduce the
+ competition of the mmap_lock.
+
+ Secondly, after using madvise() to release physical memory, there is no
+ need to build vma and allocate page tables again when accessing the same
+ virtual address again, which can also save some time.
+
I think we can use enumerated list, like below:
Thanks for your review, LGTM, will do.
-- >8 --
diff --git a/Documentation/vm/pte_ref.rst b/Documentation/vm/pte_ref.rst
index 0ac1e5a408d7c6..67b18e74fcb367 100644
--- a/Documentation/vm/pte_ref.rst
+++ b/Documentation/vm/pte_ref.rst
@@ -10,18 +10,18 @@ Preface
Now in order to pursue high performance, applications mostly use some
high-performance user-mode memory allocators, such as jemalloc or tcmalloc.
These memory allocators use madvise(MADV_DONTNEED or MADV_FREE) to release
-physical memory for the following reasons::
-
- First of all, we should hold as few write locks of mmap_lock as possible,
- since the mmap_lock semaphore has long been a contention point in the
- memory management subsystem. The mmap()/munmap() hold the write lock, and
- the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
- madvise() instead of munmap() to released physical memory can reduce the
- competition of the mmap_lock.
-
- Secondly, after using madvise() to release physical memory, there is no
- need to build vma and allocate page tables again when accessing the same
- virtual address again, which can also save some time.
+physical memory for the following reasons:
+
+1. We should hold as few write locks of mmap_lock as possible,
+ since the mmap_lock semaphore has long been a contention point in the
+ memory management subsystem. The mmap()/munmap() hold the write lock, and
+ the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
+ madvise() instead of munmap() to released physical memory can reduce the
+ competition of the mmap_lock.
+
+2. After using madvise() to release physical memory, there is no
+ need to build vma and allocate page tables again when accessing the same
+ virtual address again, which can also save some time.
The following is the largest user PTE page table memory that can be
allocated by a single user process in a 32-bit and a 64-bit system.
+The following is the largest user PTE page table memory that can be
+allocated by a single user process in a 32-bit and a 64-bit system.
+
We can say "assuming 4K page size" here,
++---------------------------+--------+---------+
+| | 32-bit | 64-bit |
++===========================+========+=========+
+| user PTE page table pages | 3 MiB | 512 GiB |
++---------------------------+--------+---------+
+| user PMD page table pages | 3 KiB | 1 GiB |
++---------------------------+--------+---------+
+
+(for 32-bit, take 3G user address space, 4K page size as an example;
+ for 64-bit, take 48-bit address width, 4K page size as an example.)
+
... instead of here.
will do.
+There is also a lock-less scenario(such as fast GUP). Fortunately, we don't need
+to do any additional operations to ensure that the system is in order. Take fast
+GUP as an example::
+
+ thread A thread B
+ fast GUP madvise(MADV_DONTNEED)
+ ======== ======================
+
+ get_user_pages_fast_only()
+ --> local_irq_save();
+ call_rcu(pte_free_rcu)
+ gup_pgd_range();
+ local_irq_restore();
+ /* do pte_free_rcu() */
+
I see whitespace warning circa do pte_free_rcu() line above when
applying this series.
will fix.
Thanks,
Qi
Thanks.
--
Thanks,
Qi
