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ARC: Pack b_byteswap in struct arc_buf_hdr #15341

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ARC: Pack b_byteswap in struct arc_buf_hdr #15341

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@amotin amotin commented Oct 3, 2023 •
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While b_byteswap rightfully belongs in struct l1arc_buf_hdr, it does not pack there efficiently. Move it to struct arc_buf_hdr allows to reduce L1 ARC header size by 8 bytes, down to 176 bytes on FreeBSD.

This PR conflicts with #15340, so for now includes it as separate commit.

Types of changes

  • Bug fix (non-breaking change which fixes an issue)
  • New feature (non-breaking change which adds functionality)
  • Performance enhancement (non-breaking change which improves efficiency)
  • Code cleanup (non-breaking change which makes code smaller or more readable)
  • Breaking change (fix or feature that would cause existing functionality to change)
  • Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
  • Documentation (a change to man pages or other documentation)

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amotin added 2 commits October 2, 2023 17:33
@amotin
ARC: Remove b_cv from struct l1arc_buf_hdr
5b4d109 
Earlier as part of openzfs#14123 I've removed one use of b_cv.  This patch
reuses the same approach to remove the other one from much more
rare code path.

This saves 16 bytes of L1 ARC header on FreeBSD (reducing it from
200 to 184 bytes) and seems even more on Linux.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
@amotin
ARC: Pack b_byteswap in struct arc_buf_hdr
7a5b5f1 
While b_byteswap rightfully belongs in struct l1arc_buf_hdr, it does
not pack there efficiently.  Move it to struct arc_buf_hdr allows to
reduce L1 ARC header size by 8 bytes, down to 176 bytes on FreeBSD.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
@amotin amotin added the Status: Code Review Needed Ready for review and testing label Oct 3, 2023
@amotin amotin mentioned this pull request Oct 3, 2023
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behlendorf
behlendorf approved these changes Oct 3, 2023
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It's a shame there isn't an efficient place to stash this in the l1arc_buf_hdr, but that does seem to be the case. Given the savings to be had by moving this it LGTM.

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amotin commented Oct 4, 2023

I am thinking may be I could remove b_bufcnt from l1arc_buf_hdr. In such case this patch won't be needed. I'll investigate this tomorrow.

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amotin commented Oct 4, 2023

In #15350 I was able to remove b_bufcnt from l1arc_buf_hdr, that allows to avoid this commit. Closing it.

@amotin amotin closed this Oct 4, 2023
@amotin amotin deleted the b_byteswap branch October 5, 2023 00:56
@amotin amotin removed the Status: Code Review Needed Ready for review and testing label Oct 5, 2023
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