avoid double alloc in NewCidV1 #132
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We allocate once via "make([]byte, len)",
and again when that buffer is converted to a string.
Thankfully, since Go 1.10 we have strings.Builder,
designed specifically for this use case.
In a downstream benchmark in go-car,
which needs to reconstruct many CID values,
we see small but nice gains:
name old time/op new time/op delta
ReadBlocks-16 1.09ms ± 4% 1.06ms ± 5% -3.33% (p=0.007 n=11+11)
name old speed new speed delta
ReadBlocks-16 478MB/s ± 4% 494MB/s ± 5% +3.46% (p=0.007 n=11+11)
name old alloc/op new alloc/op delta
ReadBlocks-16 1.30MB ± 0% 1.25MB ± 0% -3.86% (p=0.000 n=12+12)
name old allocs/op new allocs/op delta
ReadBlocks-16 9.50k ± 0% 8.45k ± 0% -11.05% (p=0.000 n=12+12)
ribasushi
approved these changes
Sep 7, 2021
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This looks sane.
Although I wonder: Multihash is a []byte, and PutUvarint woks on []byte as well. Would it not be even faster to:
- stack-allocate something reasonable ( ~80-something bytes, you need to account for obscenely large codecs+512bit hashes )
- if stuff fits - work on the above, otherwise
make()the right size - write the 1 + uvarint +
copythe mh string()the entire resulting thing
❓
|
Possibly? I guess it's a tradeoff between "copy uvarint bytes around" and "use more stack space and possibly allocate if our stack space isn't big enough". It's hard to make the call without stats. I think that copying the PutUvarint bytes (most likely just a few bytes, max 10) is cheap enough that we shouldn't pay any attention to it :) The new NewCidV1 is faster than the old one, anyway. |
Stebalien
approved these changes
Sep 12, 2021
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