rewinding blocks dialogue stays with no progress indicator or further information after hours #12030

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<!-- Describe the issue -->

Bitcoin-qt is stuck at rewinding blocks gui

<!--- What behavior did you expect? -->

progress after a few hours (ran overnight) No relevant info in debug.log except that progress number seems to be increasing vs decreasing

<!--- What was the actual behavior (provide screenshots if the issue is GUI-related)? -->

never fully loaded gui or stopped rewinding blocks dialogue

<!--- How reliably can you reproduce the issue, what are the steps to do so? -->

start bitcoin-qt either from cli or ubuntu dialogue

<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->

15.0.1 ubuntu launchpad ppa from matt corallo

<!-- What type of machine are you observing the error on (OS/CPU and disk type)? -->

acer ao722 AMD C-60 with 2Gb ram

<!-- Any extra information that might be useful in the debugging process. -->

<!--- This is normally the contents of a `debug.log` or `config.log` file. Raw text or a link to a pastebin type site are preferred. -->

If you're on a slow spinning disk on a system with memory pressure (eg 2GB ram with a web browser or other stuff running) rewinding blocks can be incredibly slow, check memory/swap usage as well as iotop to see what else is eating I/O cycles, or, since your CPU is a bit on the slow side, check top to see if its a CPU bottleneck.