对Intel Optane DC SSD用作IMDT的额外RAM感到困惑吗? [英] Confused about Intel Optane DC SSD usage as extra RAM with IMDT?
问题描述
我对Intel Optane DC感到有些困惑. 我希望我的Optane DC能够兼做DRAM和存储. 一方面，我了解到只有"Intel Optane DC持久性内存DIMM"能够用作DRAM.这是因为他具有2种模式(内存"模式和应用程序直接模式"). 另一方面，在此链接中: https://www.intel.com/content/www/us/en/products/docs/memory-storage/solid-state-drives/optane -ssd-dc-p4800x-mdt-brief.html 我读到带有Intel®内存驱动技术的DRAM和Intel®Optane™SSD共同模拟了一个易失性内存池". 我很困惑，是英特尔Optane DC SSD能够充当DRAM还是只能充当英特尔永久性内存DIMM?
I'm a little confused about Intel Optane DC. I want that my Optane DC will be able to perform as DRAM and storage both. On the one hand, I understood that only "Intel Optane DC Persistent Memory DIMM" is able to perform as DRAM.That it because he has 2 modes (Memory mode and App-Direct Mode). On the other hand, in this link: https://www.intel.com/content/www/us/en/products/docs/memory-storage/solid-state-drives/optane-ssd-dc-p4800x-mdt-brief.html I read that "Together, DRAM and Intel® Optane™ SSDs with Intel® Memory Drive Technology emulate a single volatile memory pool". I'm confused, is Intel Optane DC SSD is able to perform as DRAM or only Intel persistent Memory DIMM?
推荐答案
是的，您可以将P4800x与Intel的IMDT (英特尔内存驱动技术)软件配合使用，以给人以更多RAM的感觉通过使用Optane DC SSD作为交换空间.这就是你想要的. IMDT设置了一个虚拟机管理程序，使操作系统将DRAM + SSD当作物理内存，而不仅仅是让操作系统正常使用它作为交换空间.
Yes you can use a P4800x with Intel's IMDT (Intel Memory Drive Technology) software to give the illusion of more RAM by using the Optane DC SSD as swap space. This is what you want. IMDT sets up a hypervisor that gives the OS the illusion of DRAM + SSD as physical memory, instead of just letting the OS use it as swap space normally.
显然，当您已经有足够的物理RAM用于大多数工作集时，此方法就很好用，并且IMDT具有智能的预取算法，可以尝试在需要页面时提前分页.
Apparently this works well when you already have enough physical RAM for most of your working set, and IMDT has smart prefetching algorithms that try to page in ahead of when a page will be needed.
在IMDT虚拟机管理程序下运行操作系统而不是仅使用SSD作为交换空间的一个优点是，它将使操作系统将一些额外的空间用于页面缓存(即磁盘缓存)，而无需使用特殊代码使用(某些)SSD作为速度较慢的磁盘的缓存.
One advantage to running the OS under the IMDT hypervisor instead of just using the SSD as swap space is that it will get the OS to use some of that extra space for pagecache (aka disk caching), instead of needing special code to use (some of) an SSD as cache for a slower disk.
但是，不是，不是 Optane DC持久性存储器，那是另外一回事.
But no, it's not Optane DC Persistent Memory, that's something else.
另请参见超级用户答案 ，了解有关Optane与Optane DC PM的更多信息. Hadi Brais在其中添加了有关Optane SSD的IMDT的一些不错的部分.
See also a SuperUser answer for more about Optane vs. Optane DC PM. And Hadi Brais added some nice sections to it about IMDT for Optane SSDs.
P4800x is connected over PCI-express (as you can see in pictures on https://www.anandtech.com/show/11930/intel-optane-ssd-dc-p4800x-750gb-handson-review) for example. So it's not an NV-DIMM; you can't stick it in a DIMM socket and have the CPU access it over the memory bus. The form-factor isn't DIMM.
就硬件而言，Optane品牌名称有3种:
As far as hardware, there are 3 things with the Optane brand name:
-
消费级"Optane"固态硬盘.只是使用3D XPoint内存而不是NAND闪存的快速PCIe NVMe.
Consumer grade "Optane" SSDs. Just a fast PCIe NVMe using 3D XPoint memory instead of NAND flash.
企业"Optane DC"固态硬盘.只是使用3D XPoint内存的快速PCIe NVMe.与消费者的产品没有什么根本的区别，只是更快，更高的功耗. P4800x是这个.
Enterprise "Optane DC" SSDs. Just a fast PCIe NVMe using 3D XPoint memory. Not fundamentally different from the consumer stuff, just faster and higher power-consumption. P4800x is this.
扩展您的RAM"这里的功能是纯软件，与仅在其上创建交换分区并让OS处理对其的分页操作相比，它非常相似(并且可能更糟).尤其是如果您尚未使用虚拟化.
The "expand your RAM" functionality here is pure software, fairly similar (and possibly worse) than just creating a swap partition on it and letting the OS handle paging to it. Especially if you weren't using virtualization already.
企业"Optane DC 永久性存储器"； (简称PM).将3D XPoint内存真正(通过硬件)映射到物理地址空间，以便使用普通的加载/存储指令进行访问，而无需通过驱动程序进行每次读/写.例如Linux mmap(MAP_SYNC)，并在用户空间中使用clflush或clwb asm指令将数据提交到持久性存储.
Enterprise "Optane DC Persistent Memory" (PM for short). 3D XPoint memory that's truly mapped (by hardware) into physical address space for access with ordinary load/store instruction, without going through a driver for each read/write. e.g. Linux mmap(MAP_SYNC) and using clflush or clwb asm instructions in user-space to commit data to persistent storage.
PM仍然比DRAM慢，因此，如果您只想使用易失性内存，则仍可以将其用作IMDT之类的交换空间. DC PM的一个关键用例是使数据库能够在不通过OS的情况下提交到持久性存储.这允许围绕I/O乱序执行，并降低了开销
PM is still slower than DRAM, though, so if you just want volatile memory you might still use it as swap space like IMDT. One key use-case for DC PM is giving databases the ability to commit to persistent storage without going through the OS. This allows out-of-order execution around I/O, as well as much lower overhead.
请参见 https://www.techspot.com/news/79483-intel-announces-optane-dc-persistent-memory-dimms.html ，在传统的金字塔存储层次结构中，将Optane DC持久性存储器置于Optane DC之上.
See articles like https://www.techspot.com/news/79483-intel-announces-optane-dc-persistent-memory-dimms.html which put Optane DC Persistent Memory above Optane DC in the classic pyramid storage hierarchy.
AFAIK，Optane DC PM设备仅以DIMM形式存在，而不以PCIe形式存在(并使用类似DDR4信号的方式).这需要CPU的特殊支持，因为现代CPU集成了内存控制器.
AFAIK, Optane DC PM devices only exist in a DIMM form-factor, not PCIe (and uses something like DDR4 signalling). This requires special support from the CPU because modern CPUs integrate the memory controller.
理论上，您可以拥有一个PCIe设备，该设备在PCIe内存区域中公开一些持久性存储.这些是物理地址空间的一部分，可以配置为可回写缓存. (或者可以吗?映射MMIO区域写回不起作用)因此可以将它们映射到用户区虚拟地址空间中.但我认为不存在任何PCIe Optane DC持久性存储设备，这可能是因为PCIe命令等待时间比DDR4总线上的等待时间长得多.带宽也较低.因此，将其用作快速交换空间(在整个页面中复制)是有意义的，而不是将其用作写回可缓存的物理内存，因为在缓存中可能会等待很长时间才能丢失缓存.
In theory you could have a PCIe device that exposed some persistent storage in a PCIe memory region. Those are part of physical address space and can be configured as write-back cacheable. (Or can they? Mapping MMIO region write-back does not work) So they could be memory-mapped into userland virtual address space. But I don't think any PCIe Optane DC Persistent Memory devices exist, probably because PCIe command latency is (much) higher than over the DDR4 bus. Bandwidth is also lower. So it makes sense to use it as fast swap space (copying in a whole page), not as write-back cacheable physical memory where you could have cache misses waiting a very long time.
(玛格丽特·布鲁姆(Margaret Bloom)也评论说:写入的块大小可能是个问题.)
(Margaret Bloom also comments re: block size of writes maybe being a problem.)
即您不想要热门"商品CPU通过PCIe总线访问的内存中工作集的一部分.您甚至可能不需要Optane DC PM.
i.e. you don't want a "hot" part of your working set on memory that the CPU accesses over the PCIe bus. You probably don't even want that for Optane DC PM.
Optane/3D XPoint始终是永久性存储；究竟是要利用它还是将它用作速度较慢的易失性RAM，都取决于软件.
Optane / 3D XPoint is always persistent storage; it' up to software whether you take advantage of that or just use it as slower volatile RAM.
它不是真正意义上的DRAM，它具有特定的技术含义( dynamic =存储在需要频繁刷新的微型电容器中的数据). 3D XPoint不是动态的，甚至也不是易变的.但是您可以将其用作等效项，因为3D XPoint存储器具有非常好的写耐久性(它不会像NAND闪存那样磨损).如果人们谈论使用Optane作为更多的DRAM，他们使用的术语只是表示易失性RAM，起到了DRAM传统上所扮演的角色.
It's not literally DRAM, that has a specific technical meaning (dynamic = data stored in tiny capacitors that need refreshing frequently). 3D XPoint isn't dynamic, and isn't even volatile. But you can use it as equivalent because 3D XPoint memory has very good write endurance (it doesn't wear out like NAND flash). If people talk about using Optane as more DRAM, they're using the term to just mean volatile RAM, filling the same role that DRAM traditionally fills.
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