FreeBSD: Hardcode abd_chunk_size to PAGE_SIZE. #12328
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It makes no sense to set it below PAGE_SIZE, since it increases all overheads and makes returning memory to OS problematic. It makes no sense to set it above PAGE_SIZE, since such allocations and especially frees are too expensive and cause KVA fragmentation to benefit from fewer chunks. After that it makes no sense to keep more complicated math here. What may have sense though is just a tunable border between linear and scatter ABDs, previously also controlled by this tunable. Retain that functionality by taking abd_scatter_min_size tunable from Linux, just with different default value. Signed-off-by: Alexander Motin <mav@FreeBSD.org> Sponsored-By: iXsystems, Inc.
behlendorf
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This will help clean up the O_DIRECT PR #10018 quite a bit. I had to make specific functions for using PAGE_SIZE vs zfs_abd_chunk_size.
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It makes no sense to set it below PAGE_SIZE, since it increases all overheads and makes returning memory to OS problematic. It makes no sense to set it above PAGE_SIZE, since such allocations and especially frees are too expensive and cause KVA fragmentation to benefit from fewer chunks. After that it makes no sense to keep more complicated math here. What may have sense though is just a tunable border between linear and scatter ABDs, previously also controlled by this tunable. Retain that functionality by taking abd_scatter_min_size tunable from Linux, just with different default value. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Brian Atkinson <batkinson@lanl.gov> Reviewed-by: Ryan Moeller <ryan@iXsystems.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Closes openzfs#12328
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It makes no sense to set it below PAGE_SIZE, since it increases all overheads and makes returning memory to OS problematic. It makes no sense to set it above PAGE_SIZE, since such allocations and especially frees are too expensive and cause KVA fragmentation to benefit from fewer chunks. After that it makes no sense to keep more complicated math here. What may have sense though is just a tunable border between linear and scatter ABDs, previously also controlled by this tunable. Retain that functionality by taking abd_scatter_min_size tunable from Linux, just with different default value. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Brian Atkinson <batkinson@lanl.gov> Reviewed-by: Ryan Moeller <ryan@iXsystems.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Closes openzfs#12328
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It makes no sense to set it below PAGE_SIZE, since it increases all overheads and makes returning memory to OS problematic. It makes no sense to set it above PAGE_SIZE, since such allocations and especially frees are too expensive and cause KVA fragmentation to benefit from fewer chunks. After that it makes no sense to keep more complicated math here. What may have sense though is just a tunable border between linear and scatter ABDs, previously also controlled by this tunable. Retain that functionality by taking abd_scatter_min_size tunable from Linux, just with different default value. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Brian Atkinson <batkinson@lanl.gov> Reviewed-by: Ryan Moeller <ryan@iXsystems.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Closes #12328
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Sep 15, 2021
It makes no sense to set it below PAGE_SIZE, since it increases all overheads and makes returning memory to OS problematic. It makes no sense to set it above PAGE_SIZE, since such allocations and especially frees are too expensive and cause KVA fragmentation to benefit from fewer chunks. After that it makes no sense to keep more complicated math here. What may have sense though is just a tunable border between linear and scatter ABDs, previously also controlled by this tunable. Retain that functionality by taking abd_scatter_min_size tunable from Linux, just with different default value. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Brian Atkinson <batkinson@lanl.gov> Reviewed-by: Ryan Moeller <ryan@iXsystems.com> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Closes openzfs#12328
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It makes no sense to set it below PAGE_SIZE, since it increases all
overheads and makes returning memory to OS problematic. It makes no
sense to set it above PAGE_SIZE, since such allocations and especially
frees are too expensive and cause KVA fragmentation to benefit from
fewer chunks. After that it makes no sense to keep more complicated
math here.
What may have sense though is just a tunable border between linear and
scatter ABDs, previously also controlled by this tunable. Retain that
functionality by taking abd_scatter_min_size tunable from Linux, just
with different default value.
How Has This Been Tested?
On 80-thread FreeBSD system doing ~15GB/s of 64KB ZVOLs reads profiler shows reduction of abd_iter_*() time from 1.2% to 0.9%.
Types of changes
Checklist:
Signed-off-by.