lib,src: improve writev() performance for Buffers

[mscdex]

It's pretty common to write only Buffers to a socket, so we can optimize for that case in a couple of ways whenever we know we have all Buffers queued up for writev():

• Reuse the existing array of chunks
• Try to write the Buffers (or at least as much as possible) immediately without having to create a WriteWrap instance first. This optimization is one that is already being done when write()ing a single Buffer (or a single string for that matter) to a socket.

Benchmark results:

 improvement confidence p.value
net/net-c2s-cork.js dur=5 type="buf" len=1024 32.36 % *** 3.042522e-23
net/net-c2s-cork.js dur=5 type="buf" len=128 27.74 % *** 5.759989e-59
net/net-c2s-cork.js dur=5 type="buf" len=16 30.72 % *** 1.594866e-57
net/net-c2s-cork.js dur=5 type="buf" len=32 31.18 % *** 8.568983e-62
net/net-c2s-cork.js dur=5 type="buf" len=4 24.99 % *** 2.220787e-64
net/net-c2s-cork.js dur=5 type="buf" len=512 29.04 % *** 3.058048e-32
net/net-c2s-cork.js dur=5 type="buf" len=64 29.34 % *** 5.167685e-50
net/net-c2s-cork.js dur=5 type="buf" len=8 11.35 % *** 4.258937e-43

CI: https://ci.nodejs.org/job/node-test-pull-request/8271/
CI for FIPS (since it failed initially): https://ci.nodejs.org/job/node-test-commit-linux-fips/8624/

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

• net
• stream

[benjamingr]

I'd probably do allBuffers = allBuffers && entry.isBuf

[mscdex]

I tried something similar (continuing the rest of the loop without any conditional) and did not see any difference in performance.

[addaleax]

Can you use the overload that returns a Maybe<bool>, or alternatively use IsTrue() instead?

[mscdex]

Changed.

[addaleax]

nit: I’d really just write / 2, it’s clearer and the resulting code will be the same

[mscdex]

That's copied from before these changes. It's also similar to what was already being used in js.

[addaleax]

Yeah, but in JS that has a different meaning. ¯\_(ツ)_/¯ If you want to keep it it’s fine.

[addaleax]

Heads up, this is going to conflict with #13174 which removes the comment (because GetAsyncWrap() does actually never return a null pointer), so feel free to remove it here as well

[mscdex]

I'll just wait until that lands first and I'll rebase.

[addaleax]

@mscdex Landed, you can rebase now

[mscdex]

Done.

[addaleax]

nit: chunk->IsUint8Array()

[mscdex]

This was also copied from before these changes. I think changing it to check for a more general type is outside the scope of this PR.

[addaleax]

I think changing it to check for a more general type is outside the scope of this PR.

It’s literally the same thing, just inlined. :)

[mscdex]

Actually HasInstance() in master and v8.x use IsArrayBufferView() while v7.x and v6.x use IsUint8Array(). Ideally this could be backported more easily if we kept HasInstance()?

[addaleax]

Oh, right, we made that switch. Idc, you can also keep Buffer::HasInstance.

[MylesBorins]

Should we consider this for LTS? if so it needs to bake for a bit. Please change labels as appropriate