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The goal is to make allocation less likely to fail. With async reclaimer there is an implicit bound on the amount of memory that can be allocated between deferring points. This bound is difficult to enforce though. Sync reclaimer lifts this limitation off. Also, allocations which could not be satisfied before because of fragmentation now will have higher chances of succeeding, although depending on how much memory is fragmented, that could involve evicting a lot of segments from cache, so we should still avoid them. Downside of sync reclaiming is that now references into regions may be invalidated not only across deferring points but at any allocation site. compaction_lock can be used to pin data, preferably just temporarily.
#Urchin
##Building Urchin
In addition to required packages by Seastar, the following packages are required by Urchin.
Submodules
Urchin uses submodules, so make sure you pull the submodules first by doing:
git submodule init
git submodule update --recursive
Building urchin on Fedora
Installing required packages:
sudo yum install yaml-cpp-devel lz4-devel zlib-devel snappy-devel jsoncpp-devel thrift-devel antlr3-tool antlr3-C++-devel libasan libubsan
Building urchin on Ubuntu 14.04
Installing required packages:
sudo apt-get install libyaml-cpp-dev liblz4-dev zlib1g-dev libsnappy-dev libjsoncpp-dev