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SubjectRe: [RFC][PATCH v2 3/3] mm/zsmalloc: increase ZS_MAX_PAGES_PER_ZSPAGE
On (02/22/16 09:25), Minchan Kim wrote:
[..]
> I tempted it several times with same reason you pointed out.
> But my worry was that if we increase ZS_MAX_ZSPAGE_ORDER, zram can
> consume more memory because we need several pages chain to populate
> just a object. Even, at that time, we didn't have compaction scheme
> so fragmentation of object in zspage is huge pain to waste memory.

well, the thing is -- we end up requesting less pages after all, so
zsmalloc has better chances to survive. for example, gcc5 compilation test

BASE

168 2720 0 1 115833 115831 77222 2
190 3072 0 1 109708 109707 82281 3
202 3264 0 5 1910 1895 1528 4
254 4096 0 0 380174 380174 380174 1

Total 44 285 1621495 1618234 891703


PATCHED

192 3104 1 0 3740 3737 2860 13
194 3136 0 1 7215 7208 5550 10
197 3184 1 0 11151 11150 8673 7
199 3216 0 1 9310 9304 7315 11
200 3232 0 1 4731 4717 3735 15
202 3264 0 1 8400 8396 6720 4
206 3328 0 1 22064 22051 17927 13
207 3344 0 1 4884 4877 3996 9
208 3360 0 1 4420 4415 3640 14
211 3408 0 1 11250 11246 9375 5
212 3424 1 0 3344 3343 2816 16
214 3456 0 2 7345 7329 6215 11
217 3504 0 1 10801 10797 9258 6
219 3536 0 1 5295 5289 4589 13
222 3584 0 0 6008 6008 5257 7
223 3600 0 1 1530 1518 1350 15
225 3632 0 1 3519 3514 3128 8
228 3680 0 1 3990 3985 3591 9
230 3712 0 2 2167 2151 1970 10
232 3744 1 2 1848 1835 1694 11
234 3776 0 2 1404 1384 1296 12
235 3792 0 2 672 654 624 13
236 3808 1 2 615 592 574 14
238 3840 1 2 1120 1098 1050 15
254 4096 0 0 241824 241824 241824 1

Total 129 489 1627756 1618193 850147


that's 891703 - 850147 = 41556 less pages. or 162MB less memory used.
41556 less pages means that zsmalloc had 41556 less chances to fail.


> Now, we have compaction facility so fragment of object might not
> be a severe problem but still painful to allocate 16 pages to store
> 3408 byte. So, if we want to increase ZS_MAX_ZSPAGE_ORDER,
> first of all, we should prepare dynamic creating of sub-page of
> zspage, I think and more smart compaction to minimize wasted memory.

well, I agree, but given that we allocate less pages, do we really want to
introduce this complexity at this point?

-ss

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