Date: Wed, 13 Aug 2014 10:58:42 +0100 From: David Chisnall <theraven@FreeBSD.org> To: John Baldwin <jhb@FreeBSD.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, Adrian Chadd <adrian@freebsd.org>, freebsd-current@freebsd.org, performance@freebsd.org, "current@freebsd.org" <current@freebsd.org> Subject: Re: PostgreSQL performance on FreeBSD Message-ID: <DD742BF5-09DF-4399-B247-1E36F1CC46F5@FreeBSD.org> In-Reply-To: <201408121409.40653.jhb@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>
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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? 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. Davidhelp
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