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Optimizing Linux Memory Management for Low-latency, High-throughput Databases
- WestCoastJustin 13y agoYou can also optimize memory and cpu management through linux control groups. Oracle published a pretty good description (see: example 1: NUMA Pinning) of how to assign dedicated cpus and memory to a process or group of processes [1], but you can also read about the supporting cpuset & memory cgroups subsystems too [2, 3]. p.s. I can recently created a screencast about control groups (cgroups) for anyone interested @ http://sysadmincasts.com/episodes/14-introduction-to-linux-control-groups-cgroups http://sysadmincasts.com/episodes/14-introduction-to-linux-c... [1] http://www.oracle.com/technetwork/articles/servers-storage-admin/resource-controllers-linux-1506602.html http://www.oracle.com/technetwork/articles/servers-storage-a... [2] https://www.kernel.org/doc/Documentation/cgroups/memory.txt https://www.kernel.org/doc/Documentation/cgroups/memory.txt [3] https://www.kernel.org/doc/Documentation/cgroups/cpusets.txt https://www.kernel.org/doc/Documentation/cgroups/cpusets.txt
- MichaelGG 13y agoVery interesting note about the reclaiming. Yet another warning when transparently using a NUMA system. NUMA can be a real pain. You can get a 40% hit on direct memory access, and far worse if you're modifying a cacheline in another processor. On one of our VoIP workloads, we noticed major (250%+) increase in performance and CPU stability after splitting a very thread-intensive process into multiple processes, each set with affinity to a particular core. OSes try to help you, but it seems like they're primarily concerned with multiple processes, not huge processes like databases. Such processes should become NUMA aware and handle things themselves for best performance. It might even make sense to ask if you can split the machine on NUMA boundaries and just act like they're separate systems. RAM's getting very cheap, and RAM/core is going up faster than CPU power is (it seems to me, anyways). Also, is there a reason not to use large pages directly for the mmap'd sets if you know you're going to have them hot at all times? (I assume they read the entire file on start?)
- SEJeff 13y agoOne correction, in the Linux community, they are generally referred to as Huge Pages.
- apurvamehta 13y agoHi, post author here. > Also, is there a reason not to use large pages directly for the mmap'd sets if you know you're going to have them hot at all times? (I assume they read the entire file on start?) We could use large pages directly. But, as I mentioned in the article, the performance gains would be negligible compared to the gains that come from having things in memory in the first place. These are not very large memory systems and the page table / TLB miss overhead doesn't seem to be biting us. We are just following the mantra 'pre-mature optimization is the root of all evil' :)
- erichocean 13y agoIn my experience, most people don't know they have TLB problems because, effectively, it's always bad. It's only when you start getting to the metal to see what your hardware is actually capable of that the TLB stands out as a glaring source of inefficiency. Put another way: yeah, the TLB is making your app slow, but it's doing so always, so you don't notice. Instead, you mistakenly think your hardware is just slower than it really is.
- introspectif 13y ago"after rolling out our optimizations, we saw our error rates (ie. the proportion of slow or timed out queries) drop by up to 400%" There is some good shared knowledge in the post (unlike this comment, to be fair), but what does drop by 400% mean? If a rate drops by 100% it becomes zero. I get that. If it increases by 400%, the outcome is slightly ambiguous (do we add 400% for 500% total or do we multiply up to 400% of the original value). But a rate decreasing by 400% - am I the only person who finds that (not uncommon) expression hard to conceptualize?
- AsymetricCom 13y agoObviously, he gets -300% errors now.
- ihsw 13y agoThat's pretty good.
- erichocean 13y agoI understood it to mean that it had 1/4 the error rates they were previously seeing.
- apurvamehta 13y agoThis is exactly right :)
- caf 13y agoThen it dropped by 75%.
- apurvamehta 13y agoYes. It was a blunder. The post has been updated to reflect this.
- dllthomas 13y ago
- dllthomas 13y ago"we saw our error rates (ie. the proportion of slow or timed out queries) drop by up to 400%." Should that be 80%? Edited to add: Apparently it should be 75%, per comments elsewhere.
- apurvamehta 13y agoThanks, the 400% number is wrong. It was a last minute edit.. I should learn not to do that. I have updated the post to say that the error rates have dropped by 1/4th.
- apurvamehta 13y agoYikes. I meant that they dropped TO 1/4th the original.
- deleted 13y ago[deleted]
- krakensden 13y agoAccording to LWN, this is probably going to be automatic in the future: http://lwn.net/Articles/568870/ http://lwn.net/Articles/568870/ (subscriber-only now, will be free in a week)
- caf 13y agoIn regard to conclusion 2, there is another approach here - when you're finished with an old segment, posix_fadvise(..., POSIX_FADV_DONTNEED) can be used to drop it from the page cache.
- apurvamehta 13y agoThat would be true if we were using C++. Unfortunately, all our code is in Scala and we use Java NIO libraries to memory map our files. AFAIK, they don't give us the option on using these POSIX calls.
- pquerna 13y agoCassandra binds to posix_fadvise to do exactly this when writing out new SSTables: https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/utils/CLibrary.java#L145-L162 https://github.com/apache/cassandra/blob/trunk/src/java/org/...
- apurvamehta 13y agoWow.. that's great to know. We will definitely investigate this approach. Thanks for sharing! :)
- dhruvbird 13y ago> On small setting for Linux, one dramatic performance improvement for LinkedIn! should be... you know what it should be ;)
- Erwin 13y agoI was hit by the transparent huge pages on RHEL 6.2 in my workload. If you find our ordinary processes randomly taking up huge amounts of CPU time -- system CPU time -- when doing apparently ordinary tasks, you might be affected too. That was a real pain to diagnose when you're used to trusting the kernel not doing anything that weird. Running "perf top" helped to narrow down what the system was REALLY doing. I didn't have LI-size databases -- just a dozen Python processes allocating each perhaps 300MB and all restarting at the same time were enough to trigger it, taking 10 minutes rather than 2 seconds to start up.
- vosper 13y agoDoes the information in this article apply to VMs (specifically AWS) or is it only relevant when you're running directly on hardware?
- apurvamehta 13y agoWe did our experiments directly on hardware. I don't think that AWS VMs simulate multiple physical sockets. If they don't then this article will not apply to them.