Wetin mek di fɔs C++ (m)alɔkeshɔn na 72 KB ɔltɛm?

Di 72 KB we yu si pan yu fɔs alɔkeshɔn nɔto west mɛmori — na pefɔmɛns invɛstmɛnt. Di alɔkayshɔn de bet se yu program go mek mɔ alɔkeshɔn jisnɔ, ɛn insay klos to ɛvri rial wɔl sɛnɛriɔ, da bet de pe fayn fayn wan. Di kɔst fɔ di vayrɔyal adrɛs spɛs we dɛn nɔ yuz na essentially ziro pan di mɔdan 64-bit sistem dɛn.

Virtual Mɛmori vs. Fizik Mɛmori: Wetin Mek I Nɔ Impɔtant

Wan kɔmɔn tin we de mɔna divɛlɔpman dɛn we mit dis bihayvya fɔ di fɔs tɛm na fɔ west risɔs. If a nid 4 bayt nɔmɔ, wetin mek mi program de yuz 72 KB? di krichכl insayt na dat virtual mεmכri nכto fכshal mεmכri. We glibc ɛkstɛnd di program brek bay 72 KB, di kɛnal de ɔpdet di prɔses in vayrɔyal mɛmori map dɛn, bɔt i nɔ de bak dɛn pej dɛn de wantɛm wantɛm wit fizik RAM. Di aktual fizik pej dɛn de alɔkat pan dimand tru pej fɔlt — na we yu program rayt to wan patikyula adrɛs nɔmɔ di kɛnal de asaynd rial pej fɔ mɛmori to am.

Dis min se pan ɔl we yu prɔses in vayrɔyal saiz de go ɔp bay 72 KB, in rɛsidɛnt sɛt saiz (RSS) — di amɔnt fɔ fizik RAM we yu rili yuz — de inkrisayz bay di pej dɛn nɔmɔ we yu rili tɔch. Fɔ wan singl nyu int, dat na tipikli wan 4 KB pej, plus ɛni pej we di arena mɛtadata tek. Di ɔda vayrɔyal spɛs sidɔm de, rɛdi fɔ yuz, i nɔ de kɔst natin pas adrɛs spɛs — we yu gɛt 128 TB pan 64-bit Linux sistɛm.

Dis difrɛns impɔtant we dɛn de profayl ɛn monitar prodakshɔn aplikeshɔn dɛn. If yu de bil sɔftwɛl we nid fɔ trak rial risɔs kɔnsɔmshɔn — ilɛksɛf na SaaS bakɛnd, maykrosavis, ɔ analitiks paip layn lɛk di wan dɛn we de rɔn pan pletfɔm dɛn lɛk Mewayz fɔ biznɛs ɔpreshɔn — yu fɔ ɔltɛm monitar RSS pas vayrɔyal saiz. Tul dɛm lɛk /proc/[pid]/smaps, valgrind --tool=massif, ɛn pmap kin gi yu kɔrɛkt fizik mɛmori futprin pas fɔ mislead vayrɔyal mɛmori figa dɛm.

Aw Difrɛn Alɔkeshɔn dɛn de Handle di Fɔs Alɔkeshɔn

Di 72 KB figa na spɛsifi k to glibc in ptmaloc2. Ɔda alɔkayshɔn dɛn kin mek difrɛn trade-ɔf, ɛn di fɔs alɔkeshɔn ɔvahɛd kin difrɛn akɔdin to dat. Fɔ ɔndastand dɛn difrɛns ya valyu we yu de pik alɔkat fɔ aplikeshɔn dɛn we gɛt fɔ du wit di wok we dɛn de du.

we dɛn kɔl
• jemalloc (we Facebook, FreeBSD de yuz) — Yuz wan mɔ granular arena strɔkchɔ wit thread-lɔkal kesh dɛm. Di initial ovahed kin te fɔ bi ay (bɔku tɛm 200+ KB) bɔt i kin gi bɛtɛ mɔlti-thrɛd pefɔmɛns bikɔs ɔf di ridyus lɔk kɔntinyuɛns.
• tcmalloc (Google in Thread-Caching Malloc) — I de alɔkat wan fɔ ɛni trɛd kesh we gɛt lɛk 2 MB bay difɔlt, wit agresiv prɛ-alɔkeshɔn. Di fɔs ɔvahɛd kin ay, bɔt di smɔl alɔkeshɔn dɛn we kin apin afta dat kin rili fast.
• musl libc’s malloc — Yuz wan dizayn we simpul pasmak we biys pan mmap fɔ ɔl di alɔkeshɔn dɛn. Fɔs ɔvahɛd na smɔl (bɔku tɛm na jɔs 4 KB fɔ ɛni alɔkeshɔn), bɔt fɔ ɛni alɔkeshɔn kɔst kin ay bikɔs ɔf mɔ frɛkuɛnt sistɛm kɔl.
• mimalloc (Microsoft) — I de yuz sɛgmɛnt-bɛs alɔkeshɔn wit 64 MB sɛgmɛnt dɛn. Di fɔs alɔkeshɔn de trig 64 MB vayrɔyal rizɛvshɔn (wit minimal fizik kɔmitmɛnt), tredin adrɛs spɛs fɔ ɛksɛpshɔn lokaliti ɛn thruput.

Di chuk bitwin dɛn alɔkat ya dipen ɔl pan yu woklɔd. Fɔ lɔng-rɔn sava aplikeshɔn wit ebi ebi malti-thrɛd alɔkeshɔn, jemalloc ɔ tcmalloc tipikli autperfɔm glibc in difɔlt. Fɔ mɛmori-kɔnstrayn ɛmbaded sistɛm, musl in simpul we fɔ du am kin bɛtɛ pan ɔl we i gɛt smɔl thruput. Fɔ mɔs jenɛral-pɔpɔs dɛsktɔp ɛn sava aplikeshɔn, ptmalloc2 in 72 KB initial ɔvahɛd riprizent wan rizin difɔlt we de wok fayn witout tuning.

Tyun di Fɔs Alɔkeshɔn Biɛvhɔ

If di difɔlt 72 KB initial ɔvahɛd na tru tru prɔblɛm fɔ yu yus kes — sɔntɛm yu de spawn tawzin shɔt-layv prɔses, ɛni wan de mek wan anful alɔkeshɔn nɔmɔ — glibc de gi sɔm tunables via mallopt() ɛn di MALLOC_ famili fɔ envayrɔmɛnt vɛriɔbul dɛn.

Di M_TOP_PAD paramita de kɔntrol ɔmɔs ɛkstra mɛmori di alɔkatɔ de aks fɔ pas wetin dɛn nid wantɛm wantɛm. We yu sɛt am to 0 wit mallopt(M_TOP_PAD, 0) i de tɛl di alɔkayshɔn fɔ aks fɔ ɔl wetin dɛn nid, we de ridyus di fɔs ɔvahɛd bad bad wan. Di M_MMAP_THRESHOLD paramita de kɔntrol di sayz ɔp we di alɔkeshɔn dɛn de yuz mmap instead ɔf di arena. Di M_TRIM_THRESHOLD de kɔntrol we fri mɛmori de kam bak to di OS. Ɛn bikɔs glibc 2.26, di glibc.malloc.tcache_count ɛn glibc.malloc.tcache_max tunables de mek yu kɔntrol di trɛd kesh bihayvya.

Bɔt, wan wɔd fɔ tek tɛm: fɔ tyu dɛn paramita ya we yu nɔ tek tɛm bɛnchmak ɔlmost ɔltɛm i kin mek tin wɔs. Di difɔlt dɛn bin pik bays pan bɔku rial-wɔl prɔfayl, ɛn dɛn ripresent wan swit ples fɔ di bɔku bɔku woklɔd dɛn. If yu nɔ gɛt strɔng pruf frɔm prodakshɔn profayl se malloc ovahɛd na bɔtulnɛk — ɛn yu dɔn mɛzhɔ di impak we yu chenj dɛn gɛt — lɛf di difɔlt dɛn nɔmɔ. Premature optimization of di allocator na wan patikyula insidious fכm fכ yak shaving we dεn kכnsכm kכntlεs εnjinia awa fכ nεglijibl bεnεfit.

Wetin Dis De Tich Wi Bɔt Sistɛm Progrɛm

Di 72 KB fɔs-alɔkeshɔn mistɛri na, na in kɔr, lɛsin bɔt abstrakshɔn layers. C++ de gi yu di ilyushɔn se nyu int de alɔkat 4 bayt. Di langwej standad se na so. Yu mental model se na so. Bɔt bitwin yu kɔd ɛn di hadwae sidɔm wan stak fɔ sofistikeyt sistɛm dɛn — di C++ rɔntaym, di C laybri alɔketa, di kɛnal in vayrɔyal mɛmori sabsistim, ɛn di hadwae in MMU ɛn TLB — ɛni wan de ad in yon bihayvya, ɔptimayzeshɔn, ɛn ɔvahɛd.

Dis nɔto fɔlt. Na di ɔl pɔynt fɔ sistɛm softwe. Ɛni layt de fɔ sɔlv wan rial prɔblɛm: di alɔkeshɔn de so yu nɔ nid fɔ mek sistɛm kɔl fɔ ɛvri alɔkeshɔn. Di vayrɔyal mɛmori sistɛm de so yu nɔ nid fɔ manej fizik mɛmori dairekt wan. Di pej fɔlt handla de so mɛmori de kɔmit les ɛn efyushɔn. Ɛvri layt de tred smɔl transparency fɔ big amount fɔ pefɔmɛns ɛn kɔvinant.

Di divɛlɔpa dɛn we bil di sistɛm dɛn we dɛn kin rili abop pan, we de wok fayn pas ɔl na di wan dɛn we ɔndastand dɛn layers ya — nɔto bikɔs dɛn nid fɔ tink bɔt dɛn ɔltɛm, bɔt bikɔs we sɔntin we dɛn nɔ bin de ɛkspɛkt apin (lɛk wan mistiriɔs 72 KB alɔkeshɔn), dɛn gɛt di mental mɔdel fɔ ɔndastand wetin mek. If yu de bil rial-taym tredin sistem, gem injin, ɔ biznɛs pletfɔm we de sav tawzin pipul dɛn we de yuz am, di ebul fɔ rizin bɔt wetin yu kɔd rili de du na di sistem lɛvɛl na in de separet kɔmpiten divɛlɔpa dɛn frɔm ɛksɛpshɔn wan dɛn. Di 72 KB nɔto bɔg. Na yu alɔkatɔ de du in wok fayn fayn wan.