Date: Wed, 1 Apr 2015 12:34:09 +0000 From: "Meyer, Conrad" <conrad.meyer@isilon.com> To: Garrett Cooper <yaneurabeya@gmail.com>, "Gumpula, Suresh" <Suresh.Gumpula@netapp.com> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: RE: BSD 8.1 and 9.1 memory increase Message-ID: <A3CAF0E84A34A540B4C74454358E003F370F8673@MX103CL02.corp.emc.com> In-Reply-To: <CBDA4B8C-D0F5-43C1-9E7E-604EA7DA4BCD@gmail.com> References: <D12DE5E5.2F3FB%gsuresh@netapp.com> <D12DE5F8.2F3FE%gsuresh@netapp.com> <D140B84E.34458%gsuresh@netapp.com>, <CBDA4B8C-D0F5-43C1-9E7E-604EA7DA4BCD@gmail.com>
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> On Mar 31, 2015, at 17:54, Gumpula, Suresh <Suresh.Gumpula@netapp.com> wr= ote:=0A= >=0A= > Still trying to find out the reason for more memory foot print on 9.1=0A= > compared to 8.1 .=0A= > Does some thing like clustering changes in page fault handling cause=0A= > memory foot print ?=0A= > https://svnweb.freebsd.org/base?view=3Drevision&revision=3D235876=0A= >=0A= > Copying Alan Cox , if could throw some inputs on this.=0A= =0A= Superpages and how FreeBSD does its best to put runtime libraries in superp= age-able comes to mind..=0A= =0A= The VMEM for libraries is what caught us off guard last year when dealing w= ith applications -- more libraries =3D=3D greater footprint past either 8.0= or 9.0 because of changes to VM/rtld.=0A= =0A= Conrad Meyer had a change out to reduce the footprint for libraries, but it= was racy/incomplete unfortunately :/..=0A= =0A= Hope that maybe helps...=0A= =0A= -----------------------------------=0A= =0A= Right. So the linker and RTLD map each binary segment with 2MB virtual page= s, because that way you only need one mapping / TLB entry per segment (or a= t least, up to 2MB... most libraries are much smaller than this). This is a= performance optimization. The discussion around unmapping unused portions = of the 2MB range can be found here: https://reviews.freebsd.org/D1263 .=0A= =0A= To summarize: larger-than-necessary superpage mappings affect only vmem acc= ounting; actually use less resources (PTE's and any additional per-PTE vm a= ccounting) than 4k pages; and use fewer TLB entries. Unmapping the unused p= ortions is useless even if you get it right.=0A= =0A= Are you actually seeing greater memory footprint, or just greater vmem foot= print? I don't actually use FreeBSD8 or 9.=0A= =0A= Cheers,=0A= Conrad=
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