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61009379 MD |
1 | RCU Judy Array Design |
2 | Mathieu Desnoyers <mathieu.desnoyers@efficios.com> | |
3 | March 8, 2012 | |
4 | ||
5 | Initial ideas based on the released Judy Shop Manual | |
6 | (http://judy.sourceforge.net/). Judy was invented by Doug Baskins and | |
7 | implemented by Hewlett-Packard. | |
8 | ||
9 | Thresholds and RCU-specific analysis is introduced in this document. | |
10 | ||
11 | Advantages of using Judy Array (compressed nodes) for RCU tree: | |
12 | - no rebalancing | |
13 | - no transplant | |
14 | - RCU-friendly! | |
15 | - favor cache-line alignment of structures | |
16 | ||
17 | Disadvantage: | |
18 | - updates that need to reallocate nodes are slower than, e.g. non-rcu | |
19 | red-black trees. | |
20 | ||
21 | Choice: Using 256 entries intermediate nodes (index can be represented | |
22 | on 8 bits): 4 levels on 32-bit, 8 levels on 64-bit | |
23 | ||
24 | ||
25 | * Node types (from less dense node to most dense) | |
26 | ||
27 | ||
28 | - empty node: | |
29 | ||
30 | Parent pointer is NULL. | |
31 | ||
32 | ||
33 | - Type A: sequential search in value and pointer arrays | |
34 | ||
35 | + Add/removal just needs to update value and pointer array, single-entry | |
36 | (non-RCU...). For RCU, we might need to update the entire node anyway. | |
37 | - Requires sequential search through all value array for lookup fail | |
38 | test. | |
39 | ||
40 | Filled at 3 entries max 64-bit | |
41 | 8 bits indicating number of children | |
42 | Array of 8-bit values followed by array of associated pointers. | |
43 | 64-bit: 1 byte + 3 bytes + 4 bytes pad + 3*8 = 32 bytes | |
44 | ||
45 | -> up to this point on 64-bit, sequential lookup and pointer read fit in | |
46 | a 32-byte cache line. | |
47 | - lookup fail&success: 1 cache-line. | |
48 | ||
49 | Filled at 6 entries max 32-bit, 7 entries max 64-bit | |
50 | 8 bits indicating number of children | |
51 | Array of 8-bit values followed by array of associated pointers. | |
52 | 32-bit: 1 byte + 6 bytes + 1 byte pad + 6*4bytes = 32 bytes | |
53 | 64-bit: 1 byte + 7 bytes + 7*8 = 64 bytes | |
54 | ||
55 | -> up to this point on 32-bit, sequential lookup and pointer read fit in | |
56 | a 32-byte cache line. | |
57 | - lookup fail&success: 1 cache-line. | |
58 | ||
59 | Filled at 12 entries max 32-bit, 14 entries max 64-bit | |
60 | 8 bits indicating number of children | |
61 | Array of 8-bit values followed by array of associated pointers. | |
62 | 32-bit: 1 byte + 12 bytes + 3 bytes pad + 12*4bytes = 64 bytes | |
63 | 64-bit: 1 byte + 14 bytes + 1 byte pad + 14*8 = 128 bytes | |
64 | ||
65 | Filled at 25 entries max 32-bit, 28 entries max 64-bit | |
66 | 8 bits indicating number of children | |
67 | Array of 8-bit values followed by array of associated pointers. | |
68 | 32-bit: 1 byte + 25 bytes + 2 bytes pad + 25*4bytes = 128 bytes | |
69 | 64-bit: 1 byte + 28 bytes + 3 bytes pad + 28*8 = 256 bytes | |
70 | ||
71 | ---> up to this point, on both 32-bit and 64-bit, the sequential lookup | |
72 | in values array fits in a 32-byte cache line. | |
73 | - lookup failure: 1 cache line. | |
74 | - lookup success: 2 cache lines. | |
75 | ||
76 | The two below are listed for completeness sake, but because they require | |
77 | 2 32-byte cache lines for lookup, these are deemed inappropriate. | |
78 | ||
79 | Filled at 51 entries max 32-bit, 56 entries max 64-bit | |
80 | 8 bits indicating number of children | |
81 | Array of 8-bit values followed by array of associated pointers. | |
82 | 32-bit: 1 byte + 51 bytes + 51*4bytes = 256 bytes | |
83 | 64-bit: 1 byte + 56 bytes + 7 bytes pad + 56*8 = 512 bytes | |
84 | ||
85 | Filled at 102 entries max 32-bit, 113 entries max 64-bit | |
86 | 8 bits indicating number of children | |
87 | Array of 8-bit values followed by array of associated pointers. | |
88 | 32-bit: 1 byte + 102 bytes + 1 byte pad + 102*4bytes = 512 bytes | |
89 | 64-bit: 1 byte + 113 bytes + 6 bytes pad + 113*8 = 1024 bytes | |
90 | ||
91 | ||
fd800776 | 92 | - Type B: pools of values and pointers arrays |
61009379 | 93 | |
fd800776 MD |
94 | Pools of values and pointers arrays. Each pool values array is 32-bytes |
95 | in size (so it fits in a L1 cacheline). Each pool begins with an 8-bit | |
96 | integer, which is the number of children in this pool, followed by an | |
97 | array of 8-bit values, padding, and an array of pointers. Values and | |
98 | pointer arrays are associated as in Type A. | |
61009379 | 99 | |
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100 | The entries of a node are associated to their respective pool based |
101 | on their index position. | |
61009379 | 102 | |
fd800776 MD |
103 | + Allows lookup failure to use 1 32-byte cache-line only. (1 cacheline) |
104 | lookup success: 2 cache lines. | |
61009379 | 105 | |
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106 | + Allows in-place updates without reallocation, except when a pool is |
107 | full. (this was not possible with bitmap-based nodes) | |
108 | - If one pool exhausts its space, we need to increase the node size. | |
109 | Therefore, for very dense populations, we will end up using the | |
110 | pigeon-hole node type sooner, thus consuming more space. | |
61009379 | 111 | |
fd800776 | 112 | Pool configuration: |
61009379 | 113 | |
fd800776 MD |
114 | Per pool, filled at 25 entries (32-bit), 28 entries (64-bit) |
115 | 32-bit: 1 byte + 25 bytes + 2 bytes pad + 25*4bytes = 128 bytes | |
116 | 64-bit: 1 byte + 28 bytes + 3 bytes pad + 28*8 = 256 bytes | |
117 | ||
118 | Total up to 50 entries (32-bit), 56 entries (64-bit) | |
119 | 2 pools: 32-bit = 256 bytes | |
120 | 2 pools: 64-bit = 512 bytes | |
121 | ||
122 | Total up to 100 entries (32-bit), 112 entries (64-bit) | |
123 | 4 pools: 32-bit = 512 bytes | |
124 | 4 pools: 32-bit = 1024 bytes | |
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125 | |
126 | ||
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127 | * Choice of pool configuration distribution: |
128 | ||
129 | We have pools of either 2 or 4 linear arrays. Their total size is | |
130 | between 256 bytes (32-bit 2 arrays) and 1024 bytes (64-bit 4 arrays). | |
131 | ||
132 | Alignment on 256 bytes means that we can spare the 8 least significant | |
133 | bits of the pointers. Given that the type selection already uses 3 bits, | |
134 | we have 7 bits left. | |
135 | ||
136 | Alignment on 512 bytes -> 8 bits left. | |
137 | ||
138 | We can therefore encode which bit, or which two bits, are used as | |
139 | distribution selection. We can use this technique to reequilibrate pools | |
140 | if they become unbalanced (e.g. all children are within one of the two | |
141 | linear arrays). | |
142 | ||
143 | Assuming that finding the exact sub-pool usage maximum for any given | |
144 | distribution is NP complete (not proven). | |
145 | ||
146 | Taking into account unbalance ratios (tested programmatically by | |
147 | randomly taking N entries from 256, calculating the distribution for | |
148 | each bit (number of nodes for which bit is one/zero), and calculating | |
149 | the difference in number of nodes for each bit, choosing the minimum | |
150 | difference -- for millions of runs). | |
151 | ||
152 | tot entries unbalance largest linear array (stat. approx.) | |
153 | --------------------------------------------------------------------- | |
3d45251f MD |
154 | 48 entries: 2 (98%) 24+1=25 (target ~50/2=25) |
155 | 54 entries: 2 (97%) 27+1=28 (target ~56/2=28) | |
fa89978a MD |
156 | |
157 | Note: there exists rare worse cases where the unbalance is larger, but | |
158 | it happens _very_ rarely. But need to provide a fallback if the subclass | |
159 | does not fit, but it does not need to be efficient. | |
160 | ||
161 | ||
162 | For pool of size 4, we need to approximate what is the maximum unbalance | |
163 | we can get for choice of distributions grouped by pairs of bits. | |
164 | ||
165 | tot entries unbalance largest linear array (stat. approx.) | |
166 | --------------------------------------------------------------------- | |
3d45251f MD |
167 | 92 entries: 8 (99%) 23+2=25 (target: ~100/4=25) |
168 | 104 entries: 8 (99%) 26+2=28 (target: ~112/4=28) | |
fa89978a MD |
169 | |
170 | ||
171 | Note: there exists rare worse cases where the unbalance is larger, but | |
172 | it happens _very_ rarely. But need to provide a fallback if the subclass | |
173 | does not fit, but it does not need to be efficient. | |
174 | ||
175 | ||
176 | * Population "does not fit" and distribution fallback | |
177 | ||
178 | When adding a child to a distribution node, if the child does not fit, | |
179 | we recalculate the best distribution. If it does not fit in that | |
180 | distribution neither, we need to expand the node type. | |
181 | ||
182 | When removing a child, if the node child count is brought to the number | |
183 | of entries expected to statistically fit in the lower order node, we try | |
184 | to shrink. However, if we notice that the distribution does not actually | |
185 | fit in that shrinked node, we abort the shrink operation. If shrink | |
186 | fails, we keep a counter of insertion/removal operations on the node | |
187 | before we allow the shrink to be attempted again. | |
188 | ||
189 | ||
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190 | - Type C: pigeon-hole array |
191 | ||
192 | Filled at 47.2%/48.8% or more (32-bit: 121 entries+, 64-bit: 125 entries+) | |
193 | Array of children node pointers. Pointers NULL if no child at index. | |
194 | 32-bit: 4*256 = 1024 bytes | |
195 | 64-bit: 8*256 = 2048 bytes | |
196 | ||
197 | ||
198 | * Analysis of the thresholds: | |
199 | ||
200 | Analysis of number of cache-lines touched for each node, per-node-type, | |
201 | depending on the number of children per node, as we increment the number | |
202 | of children from 0 to 256. Through this, we choose number of children | |
203 | thresholds at which it is worthwhile to use a different node type. | |
204 | ||
205 | Per node: | |
206 | ||
207 | - ALWAYS 1 cache line hit for lookup failure (all cases) | |
208 | ||
209 | 32-bit | |
210 | ||
211 | - Unexisting | |
212 | ||
213 | 0 children | |
214 | ||
215 | - Type A: sequential search in value and pointer arrays | |
216 | - 1 cache line hit for lookup success | |
217 | - 32 bytes storage | |
218 | ||
219 | up to 6 children | |
220 | ||
221 | - 2 cache line hit for lookup success | |
222 | - 64 bytes storage | |
223 | ||
224 | up to 12 children | |
225 | ||
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226 | - 128 bytes storage |
227 | ||
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228 | up to 25 children |
229 | ||
230 | - Type B: pool | |
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231 | |
232 | - 256 bytes storage | |
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233 | |
234 | up to 50 children | |
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235 | |
236 | - 512 bytes storage | |
fd800776 | 237 | up to 100 children |
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238 | |
239 | - Type C: pigeon-hole array | |
240 | - 1 cache line hit for lookup success | |
241 | - 1024 bytes storage | |
242 | ||
243 | up to 256 children | |
244 | ||
245 | ||
246 | 64-bit | |
247 | ||
248 | - Unexisting | |
249 | ||
250 | 0 children | |
251 | ||
252 | - Type A: sequential search in value and pointer arrays | |
253 | - 1 cache line hit for lookup success | |
254 | - 32 bytes storage | |
255 | ||
256 | up to 3 children | |
257 | ||
258 | - 2 cache line hit for lookup success | |
259 | - 64 bytes storage | |
260 | ||
261 | up to 7 children | |
262 | ||
263 | - 128 bytes storage | |
264 | ||
265 | up to 14 children | |
266 | ||
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267 | - 256 bytes storage |
268 | ||
269 | up to 28 children | |
270 | ||
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271 | - Type B: pool |
272 | ||
61009379 | 273 | - 512 bytes storage |
fd800776 | 274 | up to 56 children |
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275 | |
276 | - 1024 bytes storage | |
fd800776 | 277 | up to 112 children |
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278 | |
279 | - Type C: pigeon-hole array | |
280 | - 1 cache line hit for lookup success | |
281 | - 2048 bytes storage | |
282 | ||
283 | up to 256 children | |
284 | ||
285 | ||
286 | * Analysis of node type encoding and node pointers: | |
287 | ||
288 | Lookups are _always_ from the top of the tree going down. This | |
289 | facilitates RCU replacement as we only keep track of pointers going | |
290 | downward. | |
291 | ||
292 | Type of node encoded in the parent's pointer. Need to reserve 2 | |
293 | least-significant bits. | |
294 | ||
295 | Types of children: | |
296 | ||
297 | enum child_type { | |
e5227865 | 298 | RCU_JA_LINEAR = 0, /* Type A */ |
fd800776 MD |
299 | /* 32-bit: 1 to 25 children, 8 to 128 bytes */ |
300 | /* 64-bit: 1 to 28 children, 16 to 256 bytes */ | |
301 | RCU_JA_POOL = 1, /* Type B */ | |
302 | /* 32-bit: 26 to 100 children, 256 to 512 bytes */ | |
303 | /* 64-bit: 29 to 112 children, 512 to 1024 bytes */ | |
e5227865 | 304 | RCU_JA_PIGEON = 2, /* Type C */ |
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305 | /* 32-bit: 101 to 256 children, 1024 bytes */ |
306 | /* 64-bit: 113 to 256 children, 2048 bytes */ | |
e5227865 | 307 | /* Leaf nodes are implicit from their height in the tree */ |
61009379 MD |
308 | }; |
309 | ||
310 | If entire pointer is NULL, children is empty. | |
311 | ||
312 | ||
313 | * Lookup and Update Algorithms | |
314 | ||
315 | Let's propose a quite simple scheme that uses a mutex on nodes to manage | |
316 | update concurrency. It's certainly not optimal in terms of concurrency | |
317 | management within a node, but it has the advantage of being simple to | |
318 | implement and understand. | |
319 | ||
320 | We need to keep a count of the number of children nodes (for each node), | |
321 | to keep track of when the node type thresholds are reached. It would be | |
322 | important to put an hysteresis loop so we don't change between node | |
323 | types too often for a loop on add/removal of the same node. | |
324 | ||
325 | We acquire locks from child to parent, nested. We take all locks | |
326 | required to perform a given update in the tree (but no more) to keep it | |
327 | consistent with respect to number of children per node. | |
328 | ||
329 | If check for node being gc'd (always under node lock) fails, we simply | |
330 | need to release the lock and lookup the node again. | |
331 | ||
332 | ||
333 | - Leaf lookup | |
334 | ||
335 | rcu_read_lock() | |
336 | ||
337 | RCU-lookup each level of the tree. If level is not populated, fail. | |
338 | Until we reach the leaf node. | |
339 | ||
340 | rcu_read_unlock() | |
341 | ||
342 | ||
343 | - Leaf insertion | |
344 | ||
345 | A) Lookup | |
346 | ||
347 | rcu_read_lock() | |
348 | RCU-lookup insert position. Find location in tree where nodes are | |
349 | missing for this insertion. If leaf is already present, insert fails, | |
350 | releasing the rcu read lock. The insert location consists of a parent | |
351 | node to which we want to attach a new node. | |
352 | ||
353 | B) Lock | |
354 | ||
355 | RCU-lookup parent node. Take the parent lock. If the parent needs to be | |
356 | reallocated to make room for this insertion, RCU-lookup parent-parent | |
357 | node and take the parent-parent lock. For each lock taken, check if | |
358 | node is being gc'd. If gc'd, release lock, re-RCU-lookup this node, and | |
359 | retry. | |
360 | ||
361 | C) Create | |
362 | ||
363 | Construct the whole branch from the new topmost intermediate node down | |
364 | to the new leaf node we are inserting. | |
365 | ||
366 | D) Populate: | |
367 | - If parent node reallocation is needed: | |
368 | Reallocate the parent node, adding the new branch to it, and | |
369 | increment its node count. | |
370 | set gc flag in old nodes. | |
371 | call_rcu free for all old nodes. | |
372 | Populate new parent node with rcu_assign_pointer. | |
373 | - Else: | |
374 | Increment parent node count. | |
375 | Use rcu_assign_pointer to populate this new branch into the parent | |
376 | node. | |
377 | ||
378 | E) Locks | |
379 | ||
380 | Release parent and (if taken) parent-parent locks. | |
381 | rcu_read_unlock() | |
382 | ||
383 | ||
384 | - Leaf removal | |
385 | ||
386 | A) Lookup | |
387 | ||
388 | rcu_read_lock() | |
389 | RCU-lookup leaf to remove. If leaf is missing, fail and release rcu | |
390 | read lock. | |
391 | ||
392 | B) Lock | |
393 | ||
394 | RCU-lookup parent. Take the parent lock. If the parent needs to be | |
395 | reallocated because it would be too large for the decremented number of | |
396 | children, RCU-lookup parent-parent and take the parent-parent lock. Do | |
397 | so recursively until no node reallocation is needed, or until root is | |
398 | reached. | |
399 | ||
400 | For each lock taken, check if node is being gc'd. If gc'd, release lock, | |
401 | re-RCU-lookup this node, and retry. | |
402 | ||
403 | C) Create | |
404 | ||
405 | The branch (or portion of branch) consisting of taken locks necessarily | |
406 | has a simple node removal or update as operation to do on its top node. | |
407 | ||
408 | If the operation is a node removal, then, necessarily, the entire branch | |
409 | under the node removal operation will simply disappear. No node | |
410 | allocation is needed. | |
411 | ||
412 | Else, if the operation is a child node reallocation, the child node will | |
413 | necessarily do a node removal. So _its_ entire child branch will | |
414 | disappear. So reallocate this child node without the removed branch | |
415 | (remember to decrement its nr children count). | |
416 | ||
417 | D) Populate | |
418 | ||
419 | No reallocation case: simply set the appropriate child pointer in the | |
420 | topmost locked node to NULL. Decrement its nr children count. | |
421 | ||
422 | Reallocation case: set the child pointer in the topmost locked node to | |
423 | the newly allocated node. | |
424 | set old nodes gc flag. | |
425 | call_rcu free for all old nodes. | |
426 | ||
427 | E) Locks | |
428 | ||
429 | Release all locks. | |
430 | rcu_read_unlock() | |
431 | ||
432 | ||
433 | For the various types of nodes: | |
434 | ||
435 | - sequential search (type A) | |
436 | - RCU replacement: mutex | |
437 | - Entry update: mutex | |
438 | ||
439 | - bitmap followed by pointer array (type B) | |
440 | - RCU replacement: mutex | |
441 | - Entry update: mutex | |
442 | ||
443 | - pigeon hole array (type C) | |
444 | - RCU replacement: mutex | |
445 | - Entry update: mutex |