Date: Sun, 2 Oct 2011 18:37:58 +0400 From: Lev Serebryakov <lev@FreeBSD.org> To: Davide Italiano <davide.italiano@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: Memory allocation in kernel -- what to use in which situation? What is the best for page-sized allocations? Message-ID: <16010671866.20111002183758@serebryakov.spb.ru> In-Reply-To: <CACYV=-EA4nG2MG73hBitgtoRQEk8d2CzwzzAf%2BbceHcqOJHuiw@mail.gmail.com> References: <358651269.20111002162109@serebryakov.spb.ru> <CACYV=-FNM-3fcYzFGc9eFajdoBmG1E-rWo6tq-OwBefGPADywA@mail.gmail.com> <1393358703.20111002174545@serebryakov.spb.ru> <CACYV=-EA4nG2MG73hBitgtoRQEk8d2CzwzzAf%2BbceHcqOJHuiw@mail.gmail.com>
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Hello, Davide. You wrote 2 =D0=BE=D0=BA=D1=82=D1=8F=D0=B1=D1=80=D1=8F 2011 =D0=B3., 18:00:= 26: >> =C2=A0BTW, I/O is often require big buffers, up to MAXPHYS (128KiB for >> =C2=A0now), do you mean, that any allocation of such memory has >> =C2=A0considerable performance penalties, especially on multi-core and >> =C2=A0multi-CPU systems? >> > In fact, the main client of such kind of allocations is the ZFS > filesystem (this is due to its mechanism of adaptative cache > replacement, ARC). Afaik, at the time in which UMA was written, such > kind of allocations you describe were so infrequent that no initial > effort was made in order to optimize them. > People tried to address this issue by having ZFS create a large number > of UMA zones for large allocations of different sizes. Unfortunately, > one of the side-effects of this approach was the growth of the > fragmentation, so we're investigating about. What about these geom modules, which allocate buffers, because need to read more, than requested by upper layer? geom_cache and geom_raid3, for example? And "my" geom_raid5 -- I begin to understand, why original author of geom_raid5 (which need MAXPHYS-sized buffers regularry) wrote its own memory management layer... --=20 // Black Lion AKA Lev Serebryakov <lev@FreeBSD.org>
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