updates with contents generation were super syscall-heavy. for each path
in a package (so at least 2-4, but ordinarily >4) we'd do a db.Put in
ContentsIndex which results in one syscall.Write. so, for every package in
a published repo we'd have to do *at least* 2 but ordinarily >4 syscalls.
this gets abysmally slow very quickly depending on the available system
specs.
instead, start a batch inside each package and finish it when we are done
with the package. this should keep the memory footprint negligible, but
reduce the write() calls from N to 1.
on one of KDE's servers I have seen update publishing of 7600 packages go
from ~28s to ~9s when using batch putting on an HDD.
on my local system the same set of packages go from ~14s to ~6s on an SSD.
(all inodes in cache in both cases)
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