From: Mathieu Desnoyers Date: Tue, 20 Mar 2012 13:15:47 +0000 (-0400) Subject: rcuja: Update design document, discuss pool distributions X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=fa89978a7592764c0d7dea094fc713dba6dec540;p=userspace-rcu.git rcuja: Update design document, discuss pool distributions Signed-off-by: Mathieu Desnoyers --- diff --git a/rcuja/design.txt b/rcuja/design.txt index 425d80d..624eb18 100644 --- a/rcuja/design.txt +++ b/rcuja/design.txt @@ -124,6 +124,69 @@ Total up to 100 entries (32-bit), 112 entries (64-bit) 4 pools: 32-bit = 1024 bytes +* Choice of pool configuration distribution: + +We have pools of either 2 or 4 linear arrays. Their total size is +between 256 bytes (32-bit 2 arrays) and 1024 bytes (64-bit 4 arrays). + +Alignment on 256 bytes means that we can spare the 8 least significant +bits of the pointers. Given that the type selection already uses 3 bits, +we have 7 bits left. + +Alignment on 512 bytes -> 8 bits left. + +We can therefore encode which bit, or which two bits, are used as +distribution selection. We can use this technique to reequilibrate pools +if they become unbalanced (e.g. all children are within one of the two +linear arrays). + +Assuming that finding the exact sub-pool usage maximum for any given +distribution is NP complete (not proven). + +Taking into account unbalance ratios (tested programmatically by +randomly taking N entries from 256, calculating the distribution for +each bit (number of nodes for which bit is one/zero), and calculating +the difference in number of nodes for each bit, choosing the minimum +difference -- for millions of runs). + +tot entries unbalance largest linear array (stat. approx.) +--------------------------------------------------------------------- +41 entries: 9 20.5+4.5=25 (target ~50/2=25) +47 entries: 9 23.5+4.5=28 (target ~56/2=28) + +Note: there exists rare worse cases where the unbalance is larger, but +it happens _very_ rarely. But need to provide a fallback if the subclass +does not fit, but it does not need to be efficient. + + +For pool of size 4, we need to approximate what is the maximum unbalance +we can get for choice of distributions grouped by pairs of bits. + +tot entries unbalance largest linear array (stat. approx.) +--------------------------------------------------------------------- +80 entries: 20 20+5=25 (target: ~100/4=25) +90 entries: 22 22.5+5.5=28 (target: ~112/4=28) + + +Note: there exists rare worse cases where the unbalance is larger, but +it happens _very_ rarely. But need to provide a fallback if the subclass +does not fit, but it does not need to be efficient. + + +* Population "does not fit" and distribution fallback + +When adding a child to a distribution node, if the child does not fit, +we recalculate the best distribution. If it does not fit in that +distribution neither, we need to expand the node type. + +When removing a child, if the node child count is brought to the number +of entries expected to statistically fit in the lower order node, we try +to shrink. However, if we notice that the distribution does not actually +fit in that shrinked node, we abort the shrink operation. If shrink +fails, we keep a counter of insertion/removal operations on the node +before we allow the shrink to be attempted again. + + - Type C: pigeon-hole array Filled at 47.2%/48.8% or more (32-bit: 121 entries+, 64-bit: 125 entries+)