On 2015-09-30 10:29, Petros Koutoupis wrote:
KSM is usually used in KVM or other userspace VM deployments, but that is by no means the only use-case. I actually use it regularly on most of my systems, and it does help in some cases (for example, I run a lot of distributed computing apps, often using multiple instances of the same app, and those don't always share memory to the degree they should, KSM helps with this).Christoph and Austin,You both have provided me with some valuable feedback. I will do what I can to clean this patch up and in turn apply the same dynamic functionality to the already in-kernel module. Also please see my replies below. On 9/29/15 9:32 AM, Austin S Hemmelgarn wrote:On 2015-09-28 12:45, Petros Koutoupis wrote:Christoph, See my replies below.... On 9/28/15 11:29 AM, Christoph Hellwig wrote:Hi Petros, On Mon, Sep 28, 2015 at 09:12:13AM -0500, Petros Koutoupis wrote:1. Unlike the already mainline ramdisk driver, RapidDisk is designed to be managed dynamically. That is, instead of configuring a fixed number of volumes and volume sizes as compile/boot time variables, RapidDisk will allow you to add, remove, and resize your RAM drive(s) at runtime. Besides, the built in module is designed to work with smaller sizes in mind while RapidDisk focuses on larger sizes that can reach to the multiple Gigabytes or even Terabytes. Much like the built in module, it will allocate pages as they are needed which allows for over provisioning (not that it is advised) of volume sizes.The ramdisk driver allows to selects sizes and count at module load load. I agree that having runtime control would be even better, but that's best done by adding a runtime interface to the existing driver instead of duplicating it.I understand the concern and I will definitely scope out this approach, although at the moment, I am not sure how both approaches will play nice together. As mentioned above, the current implementation requires the predefined number of ram drives with the specified size to be configured at boot time (or compiled into the kernel). The only wiggle room I see for runtime control is resizing individual volumes.Just because there is not code currently to do dynamic allocation/freeing of ramdisks in the current driver doesn't mean that it isn't possible, it just means that nobody has written code to do it yet. This functionality would be extremely useful (I often use ramdisks on a VM host as a small amount of very fast swap space for the virtual machines). On top of that, the deduplication would be a wonderful feature, although it may already be indirectly implemented through KSM (that is, when KSM is on and configured to scan everything, I'm not sure if it scans memory used by the ramdisks or not).To my understanding KSM is only applied to KVM deployments. One way I have seen my caching module work is users/vendors have a block device, map it to a RapidDisk RAM drive as a RAM based Write-Through caching node and in turn export it via a traditional SAN. The idea behind adding deduplication to this module is to minimize the RAM drive footprint when used as a block level cache.
The write-through caching may be worth looking into, although I think (not certain about this) that you can force the page cache to do write-through caching only, except that can only be done globally.
It would probably be better to improve upon the existing pagecache implementation anyway, ideally, I would love to see: 1. The ability to tell the page cache to claim some minimum amount of memory that only it can use. 2. The ability to easily tune cache parameters on a per-device (or even better, per-filesystem) basis. 3. Conversion to a framework that would allow for easy development and testing of different caching algorithms (although this is probably never going to happen).
Actually, most of the CPU time spent in an ARC cache is in the auto-tuning (the 'adaptive' bit), I've done testing just in userspace and SLRU (ARC without the adaptive sizing of the lists) uses only a little more CPU time than traditional LRU, somewhat less than ARC, and does a much better job of handling COW based workloads. COW is a tough workload for LRU caching (which is why ZFS uses ARC and not traditional LRU), as a read-modify-write cycle ends up with the read data not being needed ever again, which in turn means that MRU caching can be better in may cases for heavy read-write COW workloads.2. The majority of RapidDisk code focuses on the use of Volatile memory. The support for Non-Volatile memory is a bit newer and there may be some overlap here with the recently integrated pmem code. The only advantage to having this code within RapidDisk is to provide the user with the ability to manage both technologies simultaneously, through a single interface.Which really doesn't sound like a good enough reason to duplicate it.I do not disagree with your comment here. This component does not have to be patched into the mainline.3. The RapidCache component is designed around the Non-Volatile functionality of RapidDisk (hence the block-level Write-Through caching). It is also coded and optimized around the RapidDisk sizes/variables, out-of-box. It is worth noting that I am in the process of expanding this module to add deduplication support. This will leverage RapidDisk's ability to allocate pages only when needed and reduce the cache's memory footprint; making more out of less.Still needs some code comparism to our existing two caching solutions. I'd love to see you go ahead with the dynamic ramdisk configuration as this is clearly a very useful feature. A caching solution that is optimized for non-volatile memory does sound useful, but we'll still need a patch better explaining how it actually is as useful as it might sound.CORRECTION: I meant to say Volatile and NOT Non-Volatile. RapidCache is designed around Volatile memory. I guess I was a little to excited in my response and I do apologize for that. I will provide a code comparison in my next e-mail, after I go through the existing RAM drive code.To a certain extent, I see that as potentially less useful than optimized for non-volatile memory. While the current incarnation of the pagecache in Linux could stand to have some serious performance improvements (just think how fast things would be if we used ARC instead of plain LRU), it does still do it's job well for most workloads (although being able to tell the kernel to reserve some portion of memory _just_ for the pagecache would be an interesting and probably very useful feature).My only concern with an ARC is CPU utilization. A lot more is required to manage two lists.
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