colexecdisk: limit number of FDs used by a single disk-backed op

[yuzefovich]

This commit fixes an oversight in how we're computing the number of file descriptors that a single disk-backed operator can use. Previously, we would start the calculation at 1/16 of the node-wide limit, regardless of what that comes down to. If an operator cannot acquire the FDs, we have a hint suggesting to increase that node-wide limit, but then it wouldn't actually do much - the node would still support the same number of concurrent disk-spilling queries. This is now fixed by capping the number at 16 (which is the number that an operator gets with the default limit value). I don't think there is much upside in going higher - it seems more important to support higher concurrency of disk-spilling queries rather than to speed up a smaller number of queries by a percent.

Addresses: #91784.

Epic: CRDB-20535

Release note: None

[cockroach-teamcity]

This change is 

[DrewKimball]

Is there a convenient way to test that this increases the concurrency limit for disk-spilling queries? Also, do you think it would be possible to dynamically adjust this cap according to the amount of contention for FDs?

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @cucaroach)

[yuzefovich]

I confirmed manually that this increases the concurrency, but a unit test for this would be quite annoying to write, and I don't think it's worth it. Improving the usage of the FDs by dynamically adjusting the target number is a good idea - tracked in #91784, but I don't think it's a high priority issue since no user has complained about this before, thus I put it straight into the backlog.

TFTR!

bors r+

[yuzefovich]

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

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