GC takes too much time when create large buffers frequentely

[darcy]

I created a 16MB buffer for every client request, and it's a local variable, which means will be GCed soon. When I run a benchmark, the qps was much less than expected, only hundreds per second. By the cpu profiler, I found that GC used 77% of total cpu time. With --trace_gc option, there are many lines like this:

  785292 ms: Mark-sweep 9.5 (46.0) -> 9.4 (46.0) MB, 14 ms [external memory allocation limit reached] [GC in old space requested].

I've found in node src, and it's output by Heap::AdjustAmountOfExternalAllocatedMemory(), which is called in constructor and destructor of Class Buffer. And when amount_since_last_global_gc is more than external_allocation_limit_(default 16MB), CollectAllGarbage() will be executed, which means with (16MB/buf_size) times new creation of Buffer, CollectAllGarbage() will be called once, which consume 14ms. 

external_allocation_limit_ is relative with v8-option of --max_new_space_size, but max_new_space_size is not available when v8 snapshot is used, which is default true. I rebuild node with "./configure --without-snapshot", and run server with "--max_new_space_size=819200", and the performance is improved about 3 times. Although, GC is still expensive, with 30% of total cpu time.

So, you should be careful with large buffers.

[Andrew Chilton]

On 13 November 2012 23:09, darcy <free...@gmail.com> wrote:
> So, you should be careful with large buffers.

Not just careful, but try to avoid them anyway. By streaming your info
to/from disk, to/from requests or to/from external services you'll
avoid having to use large buffers completely. It's probably a good
practice to get into, rather than allocating large amounts of memory
per request.

Cheers,
Andy

--
Andrew Chilton
e: chi...@appsattic.com
w: http://appsattic.com/
t: https://twitter.com/andychilton

[darcy]

It cannot be avoid sometimes. In my case, I need to build a new block of binary data, then send to client. Even use C++ addon to build data, it's still necessary to return a Buffer for network sending.

[Chad Engler]

Is there no way that you can stream the chunks as you read them as opposed to buffering it all in memory first?

 

-Chad

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[darcy]

I'm not reading anything, but create some data and send to client connection.