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Groups > comp.lang.c++ > #83639
| From | Bonita Montero <Bonita.Montero@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: speed of unaligned accesses that cross page-boundaries |
| Date | 2022-04-19 15:17 +0200 |
| Organization | A noiseless patient Spider |
| Message-ID | <t3mcp2$h3$1@dont-email.me> (permalink) |
| References | <t17mqk$cj4$1@dont-email.me> <t1947l$1jv3$2@gioia.aioe.org> <t3leom$neh$1@dont-email.me> <t3mb81$ndi$1@gioia.aioe.org> |
Am 19.04.2022 um 14:51 schrieb Juha Nieminen: > Bonita Montero <Bonita.Montero@gmail.com> wrote: >>> IIRC both Intel and AMD got rid of the unaligned access penalty >>> at some point (relatively recently). ... >> >> No, for a long time. Unaligned accesses are quite common for >> I/O purposes. The only unaligned access that is inefficient >> is when you cross a page-boundary (almost twice the lantency). > > AFAIK Intel got rid of the penalty with the Sandy Bridge architecture > (i5-2400, i5-2500, i7-2600, i7-2700, etc). While that was 11 years ago, > that's "relatively recently" considering Intel's entire history. Maybe you're right. I just ran my unaligned benchmark on a old Phenom X4 945 under Ubuntu 20.04: 0x200: 50% 0x400: 51% 0x800: 50% 0x1000: 50% 0x2000: 50% 0x4000: 59% 0x8000: 59% 0x10000: 69% 0x20000: 95% 0x40000: 95% 0x80000: 94% 0x100000: 90% 0x200000: 92% 0x400000: 92% 0x800000: 91% 0x1000000: 92% 0x2000000: 91% 0x4000000: 92% 0x8000000: 92% The percentage is the speed of unaligned accesses related to aligned accesses at the same block-size. With larger block-sizes the share of the memory or higher level cache access time becomes a larger part of the access time and as 64 bytes are fetched for a whole cacheline and the access mostly fetches only one cacheline.
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speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 18:04 +0100
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 18:08 +0100
Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-20 18:12 +0000
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 19:36 +0100
Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-03-20 16:13 -0700
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-21 06:58 +0100
Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 14:49 +0000
Re: speed of unaligned accesses that cross page-boundaries Juha Nieminen <nospam@thanks.invalid> - 2022-03-21 05:59 +0000
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-21 08:56 +0100
Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 14:53 +0000
Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 16:59 +0000
Re: speed of unaligned accesses that cross page-boundaries Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-18 20:59 -0700
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 06:45 +0200
Re: speed of unaligned accesses that cross page-boundaries Juha Nieminen <nospam@thanks.invalid> - 2022-04-19 12:51 +0000
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 15:17 +0200
Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-04-19 14:11 +0000
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 16:15 +0200
Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-04-27 01:52 -0700
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-27 20:02 +0200
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-28 08:19 +0200
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-28 12:19 +0200
Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-04-28 16:00 -0700
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-29 05:52 +0200
Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-04 18:06 -0700
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-05 06:13 +0200
Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-04 21:27 -0700
Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-05 07:37 +0200
Re: speed of unaligned accesses that cross page-boundaries Öö Tiib <ootiib@hot.ee> - 2022-05-05 04:34 -0700
Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-05 12:16 -0700
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