FreeBSD src
04b9b7c507
The loader bcache attempts to determine whether readahead is useful, increasing or decreasing its readahead length based on whether a read could be serviced out of the cache. This resulted in two unfortunate behaviours: 1. A series of consecutive 32 kB reads are requested and bcache performs 16 kB readaheads. For each read, bcache determines that, since only the first 16 kB is already in the cache, the readahead was not useful, and keeps the readahead at the minimum (16 kB) level. 2. A series of consecutive 32 kB reads are requested and bcache starts with a 32 kB readahead resulting in a 64 kB being read on the first request. The second 32 kB request can be serviced out of the cache, and bcache responds by doubling its readahead length to 64 kB. The third 32 kB request cannot be serviced out of the cache, and bcache reduces its readahead length back down to 32 kB. The first syndrome converts a series of 32 kB reads into a series of (misaligned) 32 kB reads, while the second syndrome converts a series of 32 kB reads into a series of 64 kB reads; in both cases we do not increase the readahead length to its limit (currently 128 kB) no matter how many consecutive read requests are made. This change avoids this problem by tracking the "unconsumed readahead" length; readahead is deemed to be useful (and the read-ahead length is potentially increased) not only if a request was completely serviced out of the cache, but also if *any* of the request was serviced out of the cache and that length matches the amount of unconsumed readahead. Conversely, we now only reduce the readahead length in cases where there was unconsumed readahead data. In my testing on an EC2 c5.xlarge instance, this change reduces the boot time by roughly 120 ms. Reviewed by: imp, tsoome MFC after: 1 week Sponsored by: https://patreon.com/cperciva Differential Revision: https://reviews.freebsd.org/D32250 |
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| .cirrus-ci | ||
| .github | ||
| bin | ||
| cddl | ||
| contrib | ||
| crypto | ||
| etc | ||
| gnu | ||
| include | ||
| kerberos5 | ||
| lib | ||
| libexec | ||
| release | ||
| rescue | ||
| sbin | ||
| secure | ||
| share | ||
| stand | ||
| sys | ||
| targets | ||
| tests | ||
| tools | ||
| usr.bin | ||
| usr.sbin | ||
| .arcconfig | ||
| .arclint | ||
| .cirrus.yml | ||
| .clang-format | ||
| .gitattributes | ||
| .gitignore | ||
| COPYRIGHT | ||
| LOCKS | ||
| MAINTAINERS | ||
| Makefile | ||
| Makefile.inc1 | ||
| Makefile.libcompat | ||
| Makefile.sys.inc | ||
| ObsoleteFiles.inc | ||
| README.md | ||
| RELNOTES | ||
| UPDATING | ||
FreeBSD Source:
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| Directory | Description |
|---|---|
| bin | System/user commands. |
| cddl | Various commands and libraries under the Common Development and Distribution License. |
| contrib | Packages contributed by 3rd parties. |
| crypto | Cryptography stuff (see crypto/README). |
| etc | Template files for /etc. |
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| share | Shared resources. |
| stand | Boot loader sources. |
| sys | Kernel sources. |
sys/arch/conf |
Kernel configuration files. GENERIC is the configuration used in release builds. NOTES contains documentation of all possible entries. |
| tests | Regression tests which can be run by Kyua. See tests/README for additional information. |
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| usr.sbin | System administration commands. |
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