Grouch mumbling about computers.

Thoughts, ideas.

2017-08-18

Studying for the interview

Concepts to study / refresh

(Post in progress)

General

Decorators

Syntactic sugar for a function wrapper. When writing a wrapper you replace the original function with your wrapper, losing the docstrings, function name and other metadata in the process. To avoid this you can use the decorator functools.wraps that simply copies the metadata over ( Link )

misc

  • @staticmethod doesn't pass either the class or the instance as first arg, can be called on class.method() or instance.method()
  • @classmethod passes the class as first arg instead of the instance, can be called on class.method() or instance.method()
  • @property used to decorate a getter:

    @property
    def OurAtt(self):
        return self.__OurAtt
    
  • @<prop>.setter used to decorate a setter:

    @OurAtt.setter
    def OurAtt(self, val):
        if val < 0:
            self.__OurAtt = 0
        elif val > 1000:
            self.__OurAtt = 1000
        else:
            self.__OurAtt = val
    

    (If the property decorator exists the setter has to exist or the property will be read-only):

  • tuples and strings are immutable

  • lambda: "single expression anonymous function"
  • generator: Link
  • monkey patch: (classes are mutable) replacing a class method at runtime
  • copying objects: copy.copy or copy.deepcopy
  • GC:
    • refcounting
    • reference cycles
    • objects are assigned generations. newer objects are more likely to die. newer objects are dealt with first.

Python3 comments, docs, styling, testing

TBD

Python3 cool stuff

Link

Advanced unpacking

>>> a, b, *rest = range(4)
>>> a
0
>>> b
1
>>> rest
[2, 3]

rest can go anywhere ( a, *rest, b ; *rest, a, b )

Keyword only arguments

def f(a, b, *, option=True):
    pass

You can't call f(a, b, True) anymore, you have to explicitly set the value ( f(a, b, option=True) )

asyncio

  • Event loop manages and distributes execution of tasks
  • Coroutines similar to generators, returns value and program flow on await
  • Futures objects that represent the result of a task that may or may not have been executed

Coroutines contain yield points where we define possible points where a context switch can happen if other tasks are pending, but will not if no other task is pending.

A context switch in asyncio represents the event loop yielding the flow of control from one coroutine to the next.

Caching 1

Link

Optimistic Caching

Always reply with the memorized value, check if the TTL was exceeded later. This can return a non-answer (until the cache is populated)

Total latency

The latency for any given function will be the defined latency for the function plus the maximum latency of all of its (recursive) dependencies.

Caching 2

Link

Notes:

Memcache has limits (1MB value size, serializing keys/values is cpu intensive);

redis doesn't have these limitations (512MB max value size, keys and values can be binary data)

Tiered cache: "Can't hold many items" (max open file limits)

Max practical limit in ext4 is ~1 million

To avoid all clients hitting the DB at the same time when the cache expires

  • Use a task queue to dedup requests
  • Randomize refresh times
  • Coordinate between workers

MFU key expiring: Eventually it'll expire and all workers will start querying the slower cache tiers (maybe DoSing them). A good way to solve this is having a background process actively refreshing the MFU keys.

mmap

Map a file directly to program space; this is useful to avoid doing read() all the time and avoids copying the file to userspace.

dogpile effect

Self DoS when a MFU key expires. A possible solution is to only have the first process requesting the expired value perform the re-computation and have the rest serve stale data.

Cython

Faster code via static typing

Priority queue heap

Insert / RemoveMax = O(log n)

Properties:

  • Heap-order property (for a “MinHeap”)

    For every node X key(parent(X)) ≤ key(X)

  • minimum key always at root

  • Insert and deleteMin must maintain

heap ordering in array

heap ordering in array

MapReduce

Tornado

ZeroMQ

PostgreSQL

bussiness stuff

open app -> requests ad to exchange -> auction -> bidders (jampp)

arch

                             ┌─> Logger -> PostgreSQL Cluster
ELB -> bidder (60-230 inst) ─┼─> Cache (L3, Networked)
                             ├─> User segments -> Presto
                             └─> ML stuff

bidders


Kinesis -> Lambda  -> Kinesis -> S3 -> PrestoDB
             │       (Firehose)   │      ^
             │                    │      │
             v                    └──> Spark
           DynamoDB

Latency

The bidder runs on Linux (unknown), each node hosts an nginx instance which does reverse proxying to gunicorn.

Kernel:

  • Consider changing the value of the kernel's clock freq ( CONFIG_HZ )
  • Hardware offload for TCP checksum?
  • Is the TCP stack tuned for latency? BBR?
  • Can MTU be increased?
  • Swap?
    • Compressed swap? zswap?
  • zram?

Userspace:

  • Is nginx necessary?
    • The data travels Kernel ─> Nginx ─> Kernel ─> gunicorn
    • Could be Kernel ─> gunicorn
    • If necessary, can sendfile , tcp_nodelay , tcp_nopush help?
    • is config optimal?
  • Why gunicorn? Why not uwsgi?
  • is cpu affinity used?

General:

  • What's the standard response size? And headers?
  • libuv for tornado? See here

Benchmarking:

  • Is it done? How? can traffic be cloned to be replayed back later? (see tcpreplay )

Misc data:

  • The typical latency of a 1 Gbit/s network is about 200 us, while the latency with a Unix domain socket can be as low as 30 us. source
  • On Linux, some people can achieve better latencies by playing with process placement (taskset), cgroups, real-time priorities (chrt), NUMA configuration (numactl), or by using a low-latency kernel.