Config Huge Pages

本文介绍如何通过使用巨页内存提升KVM虚拟机性能,包括配置步骤与验证方法。

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Get a performance boost by backing your KVM guest with hugetlbfs

Support for hugetlbfs was added with the release of kvm-62 and it can give your kvm guest machine a performance boost anywhere up to 10%. KVM provides you with an option of using hugetlbfs by using the –mem-path option when starting your kvm guest machine. Just as a sidenote, the mem-path option also supports tmpfs filesystems.

 

 

What is hugetlbfs and why would you want to use it?

It is common nowadays for architectures to support more than one page size. As an example, x86 allows you to choose 4kb or 2MB pages. In the past, Linux traditionally used large pages for supporting the actual kernel image only but in kernel 2.6 linux started allowing processes to use huge pages. The whole purpose for huge page support is performance and you can get up to a 10% performance boost based on the application you’re running. Huge pages tend to give a bigger performance boost to applications that are memory and compute intensive. In the following sections, I’ll walk through the procedure for backing your kvm guest with huge pages.

 

1. Check that your kernel has support for Huge Pages

First thing you need to do is check that your kernel has support for huge pages. If you’re running a 2.6 kernel then you most likely have huge page support already. You can confirm by running the following command.

# grep HUGETLB /boot/config-`uname -r`
CONFIG_HUGETLBFS=y
CONFIG_HUGETLB_PAGE=y

I ran this command on a Fedora Linux host but it should be similar for your distribution. Your output should look similar if your kernel has hugetlbfs support enabled. 
 

2. Calculate the number of huge pages you require

On x86, large pages are 2MB in size so you need to find out how many 2MB pages you need to back your kvm guest. Let’s say you want to use 512 MB for your guest machine then divide 512 by 2 to get 256 pages. Add a few extra pages for additional memory requirements, lets say 40 pages to give a total of 296 pages. Record this value as you’ll need it later on.

 

3. Setup your hugetlbfs filesystem

Now you need to setup your hugetlbfs filesystem on your host. As root, create a directory called hugepages like so.

mkdir /hugepages

Next add an entry to your /etc/fstab so that when you reboot your computer, your hugetlbfs will mount automatically. Your entry should look like the following.

hugetlbfs       /hugepages  hugetlbfs       defaults        0 0

Because it is difficult to reserve large pages on a busy system, it’s always best to reserve your huge pages shortly after restart.

 

4. Reboot and reserve your huge pages

As mentioned in the previous section, reserving huge pages is best done right after a reboot as it is difficult to reserve large pages on a busy system. Right after your computer has rebooted and using the value you calculated above for the number of huge pages you want to reserve, execute the following command.

echo 296 >  /proc/sys/vm/nr_hugepages

If you want this to be a permanent setup , you can also add it to your /etc/rc.local script so that it is always reserved on startup. To verify that you have large pages reserved inspect the contents of the /proc/meminfo file as follows.

# tail -n 5 /proc/meminfo 
HugePages_Total:   296
HugePages_Free:    296
HugePages_Rsvd:      0
HugePages_Surp:      0
Hugepagesize:     2048 kB

Now you are ready to start backing your virtual machine with huge pages.

 

5. Start your kvm guest with huge page support

You have verified everything to this point and ready to back your virtual machine with large pages. Start your virtual machine with using the guest.xml option as follows:

  <memoryBacking>
    <hugepages/>
  </memoryBacking>

You will probably notice a performance increase in your virtual machine. You can check that you’re actually using huge pages by inspecting the /proc/meminfo file again. For example, this is what mine looked like while running a kvm guest backed with the values in this post.

# tail -n 5 /proc/meminfo 
HugePages_Total:   296
HugePages_Free:     39
HugePages_Rsvd:      0
HugePages_Surp:      0
Hugepagesize:     2048 kB

 That concludes the procedure for backing your guest with huge pages. It doesn't take that long to setup so give it a try and see if you get a performance boost.


2、KVM Huge Page Backed Memory

intel 的X86 CPU通常使用4Kb内存页,当是经过配置,也能够使用巨页(huge page):

(4MB on x86_32, 2MB on x86_64 and x86_32 PAE)

使用巨页,KVM的虚拟机的页表将使用更少的内存,并且将提高CPU的效率。最高情况下,可以提高20%的效率!

使用方法,需要三部:

mount -t hugetlbfs hugetlbfs /dev/hugepages
#保留一些内存给巨页
sysctl vm.nr_hugepages=516
#给 kvm 传递参数 hugepages
qemu-kvm - qemu-kvm -mem-path /dev/hugepages

其中第三步,也可以在配置文件里加入:

<memoryBacking>
<hugepages/>
</memoryBacking>

验证方式,当虚拟机正常启动以后,在虚拟机里查看:

cat /proc/meminfo |grep -i HugePages_Free



<think>我们有一个条件编译指令,它检查是否定义了宏CONFIG_CONT_PTE_HUGEPAGE_64K_ZRAM。 在C/C++中,#ifdef用于条件编译,即如果该宏被定义,则编译其后的代码,直到遇到#endif、#else或#elif。 这个宏的名字暗示了与以下内容相关: - CONFIG:通常表示这是配置选项(常见于Linux内核或大型项目) - CONT_PTE:可能指连续页表项(Continuous Page Table Entries) - HUGEPAGE_64K:64KB的大页(Huge Page) - ZRAM:内存压缩技术,用于虚拟内存管理 因此,这个宏可能用于启用一种特定的内存管理特性:使用64KB大页的连续页表项来优化ZRAM(压缩内存)的性能。 在Linux内核中,连续页表项(CONT_PTE)是一种优化,它允许将多个连续的普通页表项(PTE)合并为一个更大的映射,从而减少TLB(转译后备缓冲器)压力,提高内存访问效率。而64KB的大页则是这种连续映射的一个具体尺寸。 当ZRAM(一种将内存作为压缩块设备使用的技术)被启用时,这种优化可能特别有用,因为ZRAM对内存访问模式有特定要求,通过大页和连续映射可以提升压缩/解压性能。 但是,这段代码片段只有条件编译的起始(#ifdef),没有后续内容。因此,我们无法得知具体在什么情况下定义这个宏,以及它控制哪些代码。 在实际应用中,这个宏可能在内核配置文件中定义(如通过Kconfig系统),或者由构建系统根据目标平台的特性自动设置。 示例:在Kconfig中可能有如下配置项 config CONT_PTE_HUGEPAGE_64K_ZRAM bool "Enable 64K contiguous PTE hugepages for ZRAM" depends on HUGEPAGE && ZRAM help This option enables the use of 64KB huge pages using contiguous page table entries for ZRAM. This can improve performance by reducing TLB misses. 然后,在代码中,我们可以根据这个配置是否被选择来条件编译相关的优化代码。 由于没有具体的代码上下文,我们只能解释这个宏的作用和可能的背景。 §§相关问题§§ 1. 在Linux内核中,连续页表项(CONT_PTE)与透明大页(THP)有何区别与联系? 2. 为什么ZRAM技术会受益于64KB大页的使用?具体在哪些操作上会有性能提升? 3. 在ARM64架构中,实现64KB大页需要满足哪些硬件和软件条件? 4. 如何在内核配置中启用CONFIG_CONT_PTE_HUGEPAGE_64K_ZRAM?它依赖哪些其他配置选项? 5. 当启用该配置后,内核在哪些关键路径上会改变原有的ZRAM处理逻辑?</think>### 条件编译指令解析 这段代码是一个C/C++预处理指令,属于条件编译的范畴: ```c #ifdef CONFIG_CONT_PTE_HUGEPAGE_64K_ZRAM ``` #### 详细解释: 1. **`#ifdef` 指令** - 预处理条件编译指令,含义是"if defined" - 检查指定的宏(`CONFIG_CONT_PTE_HUGEPAGE_64K_ZRAM`)是否已被定义 - 如果该宏已被定义,则编译其后的代码块 - 如果未被定义,则跳过其后直到`#endif`的代码 2. **宏名称含义分解** - `CONFIG_`:表示这是一个配置选项,通常在内核或大型系统中使用 - `CONT_PTE`:可能代表"Continuous Page Table Entries"(连续页表项) - `HUGEPAGE_64K`:64KB大页(Huge Page),一种内存管理优化技术 - `ZRAM`:Linux中的压缩内存交换技术 3. **完整功能解析** 该配置项表示:**启用针对64KB大页优化的ZRAM连续页表项支持**,主要涉及: - 内存压缩技术(ZRAM) - 大页内存管理(64KB HugePages) - 页表连续分配优化(Continuous PTE) 当此选项启用时,系统会对使用64KB大页的ZRAM内存区域采用特殊的页表管理策略 #### 典型使用场景(Linux内核示例): ```c #ifdef CONFIG_CONT_PTE_HUGEPAGE_64K_ZRAM // 优化版的页表处理函数 static int zram_pte_alloc_huge(struct page *page) { // 使用连续页表项分配策略 return alloc_cont_pte(page, 64*1024); } #else // 标准页表处理函数 static int zram_pte_alloc_std(struct page *page) { return alloc_pte(page); } #endif ``` #### 配置方式: 1. **内核配置系统**(Kconfig): ```kconfig config CONT_PTE_HUGEPAGE_64K_ZRAM bool "Enable 64KB hugepage support for ZRAM with contiguous PTEs" depends on ZRAM && HUGETLB_PAGE help This option improves ZRAM performance when using 64KB hugepages by allocating contiguous page table entries. ``` 2. **Makefile控制**: ```makefile CFLAGS += -DCONFIG_CONT_PTE_HUGEPAGE_64K_ZRAM ``` #### 技术价值: 1. **性能优化**: - 减少TLB(转译后备缓冲器)未命中 - 提高内存访问局部性 - 降低页表遍历开销 2. **内存效率**: - 减少页表碎片 - 提高大内存块的分配成功率 - 优化ZRAM压缩/解压效率 3. **使用限制**: - 需要CPU支持大页(如ARM64的64KB页) - 需要内核配置支持连续页表分配 - 主要针对内存密集型应用
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