Date: Wed, 13 Aug 2014 11:23:44 -0500 From: Alan Cox <alan.l.cox@gmail.com> To: David Chisnall <theraven@freebsd.org> Cc: Adrian Chadd <adrian@freebsd.org>, performance@freebsd.org, "current@freebsd.org" <current@freebsd.org>, John Baldwin <jhb@freebsd.org>, freebsd-current <freebsd-current@freebsd.org>, Konstantin Belousov <kostikbel@gmail.com> Subject: Re: PostgreSQL performance on FreeBSD Message-ID: <CAJUyCcPomwaWk-xEdovz9nBkdgAcZ48mCvB3szWH4hWZGQExmw@mail.gmail.com> In-Reply-To: <DD742BF5-09DF-4399-B247-1E36F1CC46F5@FreeBSD.org> References: <20140627125613.GT93733@kib.kiev.ua> <20140716132938.GB93733@kib.kiev.ua> <CAJ-Vmomfp46GDdWb21g5xi34%2BK-DaCPeyrOHoELOLzRj7-dHrQ@mail.gmail.com> <201408121409.40653.jhb@freebsd.org> <DD742BF5-09DF-4399-B247-1E36F1CC46F5@FreeBSD.org>
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On Wed, Aug 13, 2014 at 4:58 AM, David Chisnall <theraven@freebsd.org> wrote: > On 12 Aug 2014, at 19:09, John Baldwin <jhb@FreeBSD.org> wrote: > > > OTOH, I have actually seen junk profiling _improve_ performance in > certain > > cases as it forces promotion of allocated pages to superpages since all > pages > > are dirtied. (I have a local hack that adds a new malloc option to > explicitly > > memset() new pages allocated via mmap() that gives the same benefit > without > > the junking overheadon each malloc() / free(), but it does increase > physical > > RAM usage.) > > Do you get the same effect by adding MAP_ALIGNED_SUPER | MAP_PREFAULT_READ > to the mmap() call in jemalloc? No. MAP_PREFAULT_READ does not allocate physical pages. It establishes mappings to pages that are already allocated. > I've been meaning to try the latter on BERI, as we spend a lot of time > bouncing back and forth between user code and the TLB miss handlers. Given > that jemalloc asks for memory in 8MB chunks (I think via a single mmap > call, although I'm not 100% certain), MAP_ALIGNED_SUPER should have little > impact on any platform. MAP_PREFAULT_READ may cause problems on machines > with limited RAM and no swap (I don't know if the VM subsystem knows that > it can safely discard a zero'd page that has been read but not written - > I'd hope so, but it's been a while since I read that code). > > It might be that we can make jemalloc autotune whether to use > MAP_PREFAULT_READ depending on some heuristic. I wonder if something as > simple as 'turn it on after the first mmap call' would be enough: programs > that don't use more than 8MB of RAM won't prefault, but after that the > wasted physical memory becomes an increasingly small percentage. > > David > > _______________________________________________ > freebsd-current@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org" >
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