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* [Bug libc/30786] New: Microbenchmarks should write to source buffers before copying
@ 2023-08-22 15:49 carlos at redhat dot com
2023-08-22 15:51 ` [Bug libc/30786] " carlos at redhat dot com
0 siblings, 1 reply; 2+ messages in thread
From: carlos at redhat dot com @ 2023-08-22 15:49 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=30786
Bug ID: 30786
Summary: Microbenchmarks should write to source buffers before
copying
Product: glibc
Version: 2.39
Status: NEW
Severity: normal
Priority: P2
Component: libc
Assignee: unassigned at sourceware dot org
Reporter: carlos at redhat dot com
CC: drepper.fsp at gmail dot com
Target Milestone: ---
The microbenchmarks do not currently write to the source buffers before
carrying out a copy operation from the source buffer to the destination buffer.
When the source buffers are not written to the mmap operation that allocated
them may use a "zero page" mapping for all of the pages in the source buffer.
This singular page copy, even though it covers an arbitrarily large buffer
size, may be optimized by the hardware.
In some microbenchmarks I've seen as much as a 2x acceleration on the x86_64
memcpy due to the zero page, and this performance drops when the anonymous
pages are written to in order to force the page to be allocated.
Suggested next steps:
* Alter benchtests/bench-string.h (alloc_buf) to always dirty the source
buffer.
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* [Bug libc/30786] Microbenchmarks should write to source buffers before copying
2023-08-22 15:49 [Bug libc/30786] New: Microbenchmarks should write to source buffers before copying carlos at redhat dot com
@ 2023-08-22 15:51 ` carlos at redhat dot com
0 siblings, 0 replies; 2+ messages in thread
From: carlos at redhat dot com @ 2023-08-22 15:51 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=30786
--- Comment #1 from Carlos O'Donell <carlos at redhat dot com> ---
The lack of writing to the source buffer also means that transparent huge pages
may impact the performance of the microbenchmark because the THP zero page is
slightly different from the non-THP zero page. When the source pages are
written to the difference between THP and non-THP configurations goes away.
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2023-08-22 15:49 [Bug libc/30786] New: Microbenchmarks should write to source buffers before copying carlos at redhat dot com
2023-08-22 15:51 ` [Bug libc/30786] " carlos at redhat dot com
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