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Re: Lisp_Marker size on 32bit systems

From: Paul Eggert
Subject: Re: Lisp_Marker size on 32bit systems
Date: Fri, 7 Sep 2018 20:04:52 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

Stefan Monnier wrote:
I think 32 byte objects aren't supposed to be much more likely to be
aligned on 32 byte boundaries than on 32B + 16B.

True, I miscalculated. If the cache line size is 64 bytes and objects are allocated on 16-byte boundaries (the case now), the probability that a randomly-placed 24-byte marker (allocated as 32 bytes) will straddle into two cache lines is (2-1)/4, or 25%. Whereas if objects are allocated in 8-byte boundaries as you're suggesting, the probability that the same marker will straddle is (3-1)/8, which is still 25%. So for this particular case the straddling issue should be a wash.

Those perf-stats also show improved I$ performance, which isn't
explained by your suggested explanation.  Similarly, they show a reduced
number of instructions.

Yes, it could well be that the 32-byte allocation is faster than the 24 partly due to some reason other than d-cache effects. Although there is a smaller percentage of cache misses in the 32-byte version, it could be that this is because the 32-byte version uses simpler code that would be faster even if the cache miss rate were the same.

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