Soma: /* Notes */

2021-09-23T23:26:05Z

Notes

← Older revision		Revision as of 23:26, 23 September 2021
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	==Notes==		==Notes==

			<pre>
			Lecture 5
			---------

			Group reports
			- so far they look mostly good
			- please try to add some structure to it
			- organizing around questions asked/topics discussed helps
			- section headings are nice
			- 1-2 pages is what most of you are turning in, which is good
			- but please use complete sentences, don't just use bullet points/phrases. It
			is just harder to understand/more ambiguous

			DSM
			What is the basic idea of distributed shared memory?
			- a process should be able to run across multiple computers
			- different threads on different hosts
			- but all sharing the same memory/address space
			- instead of multiple processes communicating over the network,
			we have one process sharing info with itself through shared memory,
			just like any multithreaded program

			- (we can just share part of a process's address space, then it is just
			like two processes sharing part of their memory on a single system)

			Do we like to program multithreaded programs?
			- in general, it is the hardest way to implement things
			- but on multicore systems, can be the fastest
			- because shared memory is a fast way to share state
			- avoid copies of data from messages

			Does this apply to a cluster of systems?
			- NO, not at all
			- because "shared memory" is an illusion implemented by COPYING DATA OVER THE NETWORK
			- so we can never be faster than just sending messages back and forth
			- so, why do DSM if not performance?
			- ease of use, or
			- legacy code

			How does distributed shared memory compare to virtual memory on a single CPU system?
			- very similar
			- basically we're swapping across the network instead of to disk
			- but data can change when it is swapped out


			How about a system with many cores?
			- very similar, except...
			- everything implemented in hardware
			- VERY fast network, ultra low latency
			- don't have to worry about network failures
			- system is dead then
			- copying has to happen all the time
			- between caches of cores and main memory

			How common do you think is DSM today in the cloud today? Why?
			- In a sense, DSM is alive and well, just on multicore systems
			- but everywhere else...not so much
			- just isn't more efficient in software
			- better to just send messages using some other abstraction

			Isn't a distributed cache a DSM?
			- not really, it is much more specialized
			- think content distribution networks (CDNs)

			How aware does a programmer need to be of DSM to use it?
			- not at all, it is transparent

			How aware to use it efficiently?
			- VERY aware, if you aren't careful performance will go into the toliet
			- and it is hard to tell you're making things slow, we don't think of memory access that way most of the time
			- think about how difficult it is to make cache-efficient code
			- add in a network and complexity goes up, as does cost of failures to do
			the right thing

			Eventual consistency vs strong consistency
			- if two nodes access the same memory, do they HAVE to see the same thing immediately?
			- if they don't, you can get away with eventual consistency and improve performance
			- but then, why use shared memory?

			If you're tuning your system, is it easier to
			- play with memory placement, DSM algorithms, or
			- optimize network usage?

			Note the trend here
			- when we try to fool the developer that there isn't a network, we have
			performance & scaling bottlenecks
			- need abstractions that are inherently network-aware
			- account for latency, bandwidth, reliability, (& security issues)
			</pre>

Soma: Created page with "==Discussion questions== * What is the basic idea of distributed shared memory? * How does distributed shared memory compare to virtual memory on a single CPU system? How ab..."

2021-09-23T22:15:06Z

Created page with "==Discussion questions== * What is the basic idea of distributed shared memory? * How does distributed shared memory compare to virtual memory on a single CPU system? How ab..."

New page

==Discussion questions==

* What is the basic idea of distributed shared memory?
* How does distributed shared memory compare to virtual memory on a single CPU system? How about a system with many cores?
* How aware does a programmer need to be of DSM to use it? How aware to use it ''efficiently''?
* What are the key mechanisms supporting DSM?
* How common do you think is DSM today in the cloud today? Why?

==Notes==

DistOS 2021F 2021-09-23 - Revision history

Soma: /* Notes */

Soma: Created page with "==Discussion questions== * What is the basic idea of distributed shared memory? * How does distributed shared memory compare to virtual memory on a single CPU system? How ab..."