Wells IV #socket/c++/threading

This is one of the longest tech interviews and one of the most enriching 🙂 even though I don’t “like” all their questions — very academic/theoretical, text-book driven, too low-level to be practical, not testing practical zbs.

Q: Any consistently reliable technique to detect stale order in your book?

Q: what’s your frame size if your tcp/ucp buffers sizes are so large?

Q: experience storing ticks?

—— C++ questions:
Q1: how can you avoid the cost of virtual function?
%%A: enum/switch or CRTP

Q1b: what’s CRTP

Q: what if your copy ctor signature is missing the “const”?
%%A: you can’t pass in temporaries or literal values. Such arguments must pass into “const &” parameter or “const” parameter. Correct.

Q: double delete?

Q: policy class? traits class?

Q: STL binders.. use case?

Q: how many types of smart pointers do you use?

Q: difference between java generics vs c++ templates?
%%A: type erasure. Java Compiler enforces many rules, but bytecode saves no info about the type argument, so we can get strange runtime errors.
%%A: template specialization. Template meta-programming
%%A (accepted): code bloat since each template instantiation is a separate chunk of code in the object file and in memory.
A: I have a dedicated blog post on this.

Q: what’s returned by a std::queue’s dequeue operation when it’s empty?
AA: undefined behavior, so we must check empty() before attempting dequeue. I believe ditto for a std::stack

Q: why template specialization?
%%A: customize behavior for this particular vector since the standard implementation is not suitable.

Q: how do you implement a thread-safe c++singleton
A: not obvious. See concurrent lazy singleton using static-local var

Q12: in a simple function I have
vector v1 = {“a”, “b”}; vector v2 = v1; cout<<….
What happens to the ctor, dtor etc?
A: 2 std::strings constructed on heap, vector constructed on stack; 2nd vector copy-constructed on stack; 2 new strings constructed on heap; vector destructors deletes all four strings
A: the actual char array is allocated only once for each actual value, due to reference counting in the current std::string implementation.

Q12b: you mean it’s interned?

Coding question: implement void remove_spaces(char * s) //modify the s char array in place. See %%efficient removeSpaces(char*) #Wells

—— threading, mostly in java
Q: What are the problems of CAS solutions?
A: too many retries. CAS is optimistic, but if there are too many concurrent writes, then the assumption is invalid.
%%A: missed update? Not a common issue so far.

%%Q: Synchronized keyword usage inside a static method?
AA: you need be explicit about the target object, like synchronized(MyClass.class)

Q21: Name 3 effects of java volatile keyword — Advanced. See 3effects@volatile ] java5.

Q21b: analyze the double-checking singleton implementation.
staticInst = new Student(); // inside synchronized block, this can assign an incomplete object’s address to staticInst variable, which will be visible to an unsynchronized reader. Solution – declare staticInst as volatile static field

—— System programming (ANSI-C) questions
Q: have you used kernel bypass?

Q5: how do you construct a Student object in a memory-mapped-file?
%%A: placement new

Q5b: what if we close the file and map it again in the same process?
%%A: the ptr I got from malloc is now a dangling ptr. Content will be intact, as in Quest

Q5c: what if Student has virtual functions and has a vptr.
%%A: I see no problem.

Q: you mentioned non-blocking TCP send(), so when your send fails, what can you do?
%%A: retry after completing some other tasks.

Q4: why is UDP faster than TCP
%%A: no buffering; smaller envelopes; no initial handshake; no ack;
%%A: perhaps most important for market data — multiple recipient with TCP is very heavy on sender and on network

Q4b: why buffering in tcp?
%%A: resend

Q: can tcp client use bind()?
%%A: bind to a specific client port, rather than a random port

Q6: is there socket buffer overflow in TCP?
A: probably not

Q6b: what if I try to send a big file by TCP, and the receiver’s buffer is full? Will sender care about it? Is that reliable transmission?
A: Aquis sends 100MB file. See no overflow]TCP slow receiver #non-blocking sender

Q: in your ticking risk system, how does the system get notified with new market data?
%%A: we use polling instead of notification. The use case doesn’t require notification.

Q: any network engineering techniques?

Q: what kernel parameters did you tune?

 

—— Stupid best-practice questions:
Q: what’s the benefit of IOC?

Q: Fitnesse, mock objects

Question on j4 factory. See separate blog post.

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s