When memory is not enough | Hacker News Reader

When memory is not enough

avd.reduct.ru

28 pointsdzeban6 comments

I think that memory limited apps are more common, at least when they're bootstrapped. Look at Digital Ocean, Linode or DreamHost. You get a VPS for $20 bucks with 2 GB of ram. My phone has 2 GB. My laptops have 16 GB. Life is different on these types of machines.

This makes picking tech rather difficult. On the one hand C++ or D would work well here since they both offer good memory management (D's hybrid approach is nice). They are incredibly heavy (and D confuses me. The collection wars still come: up Tango vs Phoebe). So i don't know how productive I'd be given that I'm the only person in my startup. On the other hand the JVM can tune itself and even Clojure so maybe memory wouldn't be an issue for a higher level language.

Picking a database is hard too. I want to use NoSQL (wrote a book on it, should use it). I like ArangoDB. Sadly, like MongoDB it is mmapped. There are no efficient, swanky memory management tricks like you get in an older model SQL engine. But it makes sharding/replicating a breeze. So rather than having to tweak the crap out of MySQL or Postgres and get into the engine replacement (eventually), I take a hit up front. I just have to vertically scale ArangoDB for a while until it makes more sense to replicate/shard.

I made my own comment.. but even with GC based environments, if you can control when GC happens you can set it up to run more frequently than the system controls in place. My node processing scripts tend to stay under 50mb (usually closer to 20), and services are usually pretty light as well.

You don't necessarily need to resort to a lower level language to avoid using too much memory.

Fair points...D confuses me too!

For node.js processes, like importer/exporter scripts and other timed scripts, I actually use the command line flag to expose garbage collection, and manually call after each item... it keeps the memory allocation in check (20mb instead of climbing over several hundred before done processing), and it does a pretty nice job.

For services, I'd be more inclined to not call it as much, but given that all timed event scripts run on one server currently, and potentially simultaneously, I prefer it this way.

Even with gc based environments one should check on their actual memory usage. I worked on a game engine backend in .Net that would actually hang up for several seconds when GC ran... forcing GC more often kept things running a lot smoother. A fraction of a second for my use was an acceptable lag, but several seconds wasn't.

Bit of a tangent but, I don't know if a segfault should be considered a consequence of a "naive" implementation. Seems more like a consequence of a broken one.

A simple if (!A) goto err_free; would turn that segfault into a "Failed to parse file"

Also, it looks like qsort is implemented on his system as qsort_r:

  http://osxr.org/glibc/source/stdlib/msort.c

Qsort_r appears to allocate O(n) extra memory when the data fits into 1/4 of physical memory. The comments suggest that the regular qsort is slower.

I guess, you're right - for naive implementation it should fall with error, not a segfault. Maybe I just add a check for realloc result.

As for qsort implementation - I was confused by glibc sources ;-) What I've found was a slow but in-place implementation, and what you've shown me is an actual implementation that invokes an in-place variant when allocation was failed:

  tmp = malloc (size);
  __set_errno (save);
  if (tmp == NULL)
    {
      /* Couldn't get space, so use the slower algorithm
         that doesn't need a temporary array.  */
      _quicksort (b, n, s, cmp, arg);
      return;
    }

Thanks for feedback!

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