Based on kernel version 4.1. Page generated on 2015-06-28 12:15 EST.
1 Frontswap provides a "transcendent memory" interface for swap pages. 2 In some environments, dramatic performance savings may be obtained because 3 swapped pages are saved in RAM (or a RAM-like device) instead of a swap disk. 4 5 (Note, frontswap -- and cleancache (merged at 3.0) -- are the "frontends" 6 and the only necessary changes to the core kernel for transcendent memory; 7 all other supporting code -- the "backends" -- is implemented as drivers. 8 See the LWN.net article "Transcendent memory in a nutshell" for a detailed 9 overview of frontswap and related kernel parts: 10 https://lwn.net/Articles/454795/ ) 11 12 Frontswap is so named because it can be thought of as the opposite of 13 a "backing" store for a swap device. The storage is assumed to be 14 a synchronous concurrency-safe page-oriented "pseudo-RAM device" conforming 15 to the requirements of transcendent memory (such as Xen's "tmem", or 16 in-kernel compressed memory, aka "zcache", or future RAM-like devices); 17 this pseudo-RAM device is not directly accessible or addressable by the 18 kernel and is of unknown and possibly time-varying size. The driver 19 links itself to frontswap by calling frontswap_register_ops to set the 20 frontswap_ops funcs appropriately and the functions it provides must 21 conform to certain policies as follows: 22 23 An "init" prepares the device to receive frontswap pages associated 24 with the specified swap device number (aka "type"). A "store" will 25 copy the page to transcendent memory and associate it with the type and 26 offset associated with the page. A "load" will copy the page, if found, 27 from transcendent memory into kernel memory, but will NOT remove the page 28 from transcendent memory. An "invalidate_page" will remove the page 29 from transcendent memory and an "invalidate_area" will remove ALL pages 30 associated with the swap type (e.g., like swapoff) and notify the "device" 31 to refuse further stores with that swap type. 32 33 Once a page is successfully stored, a matching load on the page will normally 34 succeed. So when the kernel finds itself in a situation where it needs 35 to swap out a page, it first attempts to use frontswap. If the store returns 36 success, the data has been successfully saved to transcendent memory and 37 a disk write and, if the data is later read back, a disk read are avoided. 38 If a store returns failure, transcendent memory has rejected the data, and the 39 page can be written to swap as usual. 40 41 If a backend chooses, frontswap can be configured as a "writethrough 42 cache" by calling frontswap_writethrough(). In this mode, the reduction 43 in swap device writes is lost (and also a non-trivial performance advantage) 44 in order to allow the backend to arbitrarily "reclaim" space used to 45 store frontswap pages to more completely manage its memory usage. 46 47 Note that if a page is stored and the page already exists in transcendent memory 48 (a "duplicate" store), either the store succeeds and the data is overwritten, 49 or the store fails AND the page is invalidated. This ensures stale data may 50 never be obtained from frontswap. 51 52 If properly configured, monitoring of frontswap is done via debugfs in 53 the /sys/kernel/debug/frontswap directory. The effectiveness of 54 frontswap can be measured (across all swap devices) with: 55 56 failed_stores - how many store attempts have failed 57 loads - how many loads were attempted (all should succeed) 58 succ_stores - how many store attempts have succeeded 59 invalidates - how many invalidates were attempted 60 61 A backend implementation may provide additional metrics. 62 63 FAQ 64 65 1) Where's the value? 66 67 When a workload starts swapping, performance falls through the floor. 68 Frontswap significantly increases performance in many such workloads by 69 providing a clean, dynamic interface to read and write swap pages to 70 "transcendent memory" that is otherwise not directly addressable to the kernel. 71 This interface is ideal when data is transformed to a different form 72 and size (such as with compression) or secretly moved (as might be 73 useful for write-balancing for some RAM-like devices). Swap pages (and 74 evicted page-cache pages) are a great use for this kind of slower-than-RAM- 75 but-much-faster-than-disk "pseudo-RAM device" and the frontswap (and 76 cleancache) interface to transcendent memory provides a nice way to read 77 and write -- and indirectly "name" -- the pages. 78 79 Frontswap -- and cleancache -- with a fairly small impact on the kernel, 80 provides a huge amount of flexibility for more dynamic, flexible RAM 81 utilization in various system configurations: 82 83 In the single kernel case, aka "zcache", pages are compressed and 84 stored in local memory, thus increasing the total anonymous pages 85 that can be safely kept in RAM. Zcache essentially trades off CPU 86 cycles used in compression/decompression for better memory utilization. 87 Benchmarks have shown little or no impact when memory pressure is 88 low while providing a significant performance improvement (25%+) 89 on some workloads under high memory pressure. 90 91 "RAMster" builds on zcache by adding "peer-to-peer" transcendent memory 92 support for clustered systems. Frontswap pages are locally compressed 93 as in zcache, but then "remotified" to another system's RAM. This 94 allows RAM to be dynamically load-balanced back-and-forth as needed, 95 i.e. when system A is overcommitted, it can swap to system B, and 96 vice versa. RAMster can also be configured as a memory server so 97 many servers in a cluster can swap, dynamically as needed, to a single 98 server configured with a large amount of RAM... without pre-configuring 99 how much of the RAM is available for each of the clients! 100 101 In the virtual case, the whole point of virtualization is to statistically 102 multiplex physical resources across the varying demands of multiple 103 virtual machines. This is really hard to do with RAM and efforts to do 104 it well with no kernel changes have essentially failed (except in some 105 well-publicized special-case workloads). 106 Specifically, the Xen Transcendent Memory backend allows otherwise 107 "fallow" hypervisor-owned RAM to not only be "time-shared" between multiple 108 virtual machines, but the pages can be compressed and deduplicated to 109 optimize RAM utilization. And when guest OS's are induced to surrender 110 underutilized RAM (e.g. with "selfballooning"), sudden unexpected 111 memory pressure may result in swapping; frontswap allows those pages 112 to be swapped to and from hypervisor RAM (if overall host system memory 113 conditions allow), thus mitigating the potentially awful performance impact 114 of unplanned swapping. 115 116 A KVM implementation is underway and has been RFC'ed to lkml. And, 117 using frontswap, investigation is also underway on the use of NVM as 118 a memory extension technology. 119 120 2) Sure there may be performance advantages in some situations, but 121 what's the space/time overhead of frontswap? 122 123 If CONFIG_FRONTSWAP is disabled, every frontswap hook compiles into 124 nothingness and the only overhead is a few extra bytes per swapon'ed 125 swap device. If CONFIG_FRONTSWAP is enabled but no frontswap "backend" 126 registers, there is one extra global variable compared to zero for 127 every swap page read or written. If CONFIG_FRONTSWAP is enabled 128 AND a frontswap backend registers AND the backend fails every "store" 129 request (i.e. provides no memory despite claiming it might), 130 CPU overhead is still negligible -- and since every frontswap fail 131 precedes a swap page write-to-disk, the system is highly likely 132 to be I/O bound and using a small fraction of a percent of a CPU 133 will be irrelevant anyway. 134 135 As for space, if CONFIG_FRONTSWAP is enabled AND a frontswap backend 136 registers, one bit is allocated for every swap page for every swap 137 device that is swapon'd. This is added to the EIGHT bits (which 138 was sixteen until about 2.6.34) that the kernel already allocates 139 for every swap page for every swap device that is swapon'd. (Hugh 140 Dickins has observed that frontswap could probably steal one of 141 the existing eight bits, but let's worry about that minor optimization 142 later.) For very large swap disks (which are rare) on a standard 143 4K pagesize, this is 1MB per 32GB swap. 144 145 When swap pages are stored in transcendent memory instead of written 146 out to disk, there is a side effect that this may create more memory 147 pressure that can potentially outweigh the other advantages. A 148 backend, such as zcache, must implement policies to carefully (but 149 dynamically) manage memory limits to ensure this doesn't happen. 150 151 3) OK, how about a quick overview of what this frontswap patch does 152 in terms that a kernel hacker can grok? 153 154 Let's assume that a frontswap "backend" has registered during 155 kernel initialization; this registration indicates that this 156 frontswap backend has access to some "memory" that is not directly 157 accessible by the kernel. Exactly how much memory it provides is 158 entirely dynamic and random. 159 160 Whenever a swap-device is swapon'd frontswap_init() is called, 161 passing the swap device number (aka "type") as a parameter. 162 This notifies frontswap to expect attempts to "store" swap pages 163 associated with that number. 164 165 Whenever the swap subsystem is readying a page to write to a swap 166 device (c.f swap_writepage()), frontswap_store is called. Frontswap 167 consults with the frontswap backend and if the backend says it does NOT 168 have room, frontswap_store returns -1 and the kernel swaps the page 169 to the swap device as normal. Note that the response from the frontswap 170 backend is unpredictable to the kernel; it may choose to never accept a 171 page, it could accept every ninth page, or it might accept every 172 page. But if the backend does accept a page, the data from the page 173 has already been copied and associated with the type and offset, 174 and the backend guarantees the persistence of the data. In this case, 175 frontswap sets a bit in the "frontswap_map" for the swap device 176 corresponding to the page offset on the swap device to which it would 177 otherwise have written the data. 178 179 When the swap subsystem needs to swap-in a page (swap_readpage()), 180 it first calls frontswap_load() which checks the frontswap_map to 181 see if the page was earlier accepted by the frontswap backend. If 182 it was, the page of data is filled from the frontswap backend and 183 the swap-in is complete. If not, the normal swap-in code is 184 executed to obtain the page of data from the real swap device. 185 186 So every time the frontswap backend accepts a page, a swap device read 187 and (potentially) a swap device write are replaced by a "frontswap backend 188 store" and (possibly) a "frontswap backend loads", which are presumably much 189 faster. 190 191 4) Can't frontswap be configured as a "special" swap device that is 192 just higher priority than any real swap device (e.g. like zswap, 193 or maybe swap-over-nbd/NFS)? 194 195 No. First, the existing swap subsystem doesn't allow for any kind of 196 swap hierarchy. Perhaps it could be rewritten to accommodate a hierarchy, 197 but this would require fairly drastic changes. Even if it were 198 rewritten, the existing swap subsystem uses the block I/O layer which 199 assumes a swap device is fixed size and any page in it is linearly 200 addressable. Frontswap barely touches the existing swap subsystem, 201 and works around the constraints of the block I/O subsystem to provide 202 a great deal of flexibility and dynamicity. 203 204 For example, the acceptance of any swap page by the frontswap backend is 205 entirely unpredictable. This is critical to the definition of frontswap 206 backends because it grants completely dynamic discretion to the 207 backend. In zcache, one cannot know a priori how compressible a page is. 208 "Poorly" compressible pages can be rejected, and "poorly" can itself be 209 defined dynamically depending on current memory constraints. 210 211 Further, frontswap is entirely synchronous whereas a real swap 212 device is, by definition, asynchronous and uses block I/O. The 213 block I/O layer is not only unnecessary, but may perform "optimizations" 214 that are inappropriate for a RAM-oriented device including delaying 215 the write of some pages for a significant amount of time. Synchrony is 216 required to ensure the dynamicity of the backend and to avoid thorny race 217 conditions that would unnecessarily and greatly complicate frontswap 218 and/or the block I/O subsystem. That said, only the initial "store" 219 and "load" operations need be synchronous. A separate asynchronous thread 220 is free to manipulate the pages stored by frontswap. For example, 221 the "remotification" thread in RAMster uses standard asynchronous 222 kernel sockets to move compressed frontswap pages to a remote machine. 223 Similarly, a KVM guest-side implementation could do in-guest compression 224 and use "batched" hypercalls. 225 226 In a virtualized environment, the dynamicity allows the hypervisor 227 (or host OS) to do "intelligent overcommit". For example, it can 228 choose to accept pages only until host-swapping might be imminent, 229 then force guests to do their own swapping. 230 231 There is a downside to the transcendent memory specifications for 232 frontswap: Since any "store" might fail, there must always be a real 233 slot on a real swap device to swap the page. Thus frontswap must be 234 implemented as a "shadow" to every swapon'd device with the potential 235 capability of holding every page that the swap device might have held 236 and the possibility that it might hold no pages at all. This means 237 that frontswap cannot contain more pages than the total of swapon'd 238 swap devices. For example, if NO swap device is configured on some 239 installation, frontswap is useless. Swapless portable devices 240 can still use frontswap but a backend for such devices must configure 241 some kind of "ghost" swap device and ensure that it is never used. 242 243 5) Why this weird definition about "duplicate stores"? If a page 244 has been previously successfully stored, can't it always be 245 successfully overwritten? 246 247 Nearly always it can, but no, sometimes it cannot. Consider an example 248 where data is compressed and the original 4K page has been compressed 249 to 1K. Now an attempt is made to overwrite the page with data that 250 is non-compressible and so would take the entire 4K. But the backend 251 has no more space. In this case, the store must be rejected. Whenever 252 frontswap rejects a store that would overwrite, it also must invalidate 253 the old data and ensure that it is no longer accessible. Since the 254 swap subsystem then writes the new data to the read swap device, 255 this is the correct course of action to ensure coherency. 256 257 6) What is frontswap_shrink for? 258 259 When the (non-frontswap) swap subsystem swaps out a page to a real 260 swap device, that page is only taking up low-value pre-allocated disk 261 space. But if frontswap has placed a page in transcendent memory, that 262 page may be taking up valuable real estate. The frontswap_shrink 263 routine allows code outside of the swap subsystem to force pages out 264 of the memory managed by frontswap and back into kernel-addressable memory. 265 For example, in RAMster, a "suction driver" thread will attempt 266 to "repatriate" pages sent to a remote machine back to the local machine; 267 this is driven using the frontswap_shrink mechanism when memory pressure 268 subsides. 269 270 7) Why does the frontswap patch create the new include file swapfile.h? 271 272 The frontswap code depends on some swap-subsystem-internal data 273 structures that have, over the years, moved back and forth between 274 static and global. This seemed a reasonable compromise: Define 275 them as global but declare them in a new include file that isn't 276 included by the large number of source files that include swap.h. 277 278 Dan Magenheimer, last updated April 9, 2012