performance - five minutes rules - the price of one access of disc I/O -

-

this interesting topic, use following formula compute access interval time:

breakevenintervalinseconds = (pagespermbofram / accessespersecondperdisk) × (priceperdiskdrive / pricepermbofram).

it derived using formulas cost of ram hold page in buffer pool , the cost of (fractional) disk perform i/o every time page needed, equating these 2 costs, , solving equation interval between accesses.

so cost of disc i/o per access priceperdiskdrive / accessespersecondperdisk, question why disc i/o cost per access computed this?

the underlying assumption limit life of disk how many disk seeks there are, while ram has fixed cost size, , fixed lifetime regardless of how accessed. reasonable because seeking disk causes physical wear , tear, , when disk goes, lose whole disk. contrast ram has no physical moving parts, , not wear out use.

with assumption, cost of keeping data on disk depends on frequency of access , cost of disk. cost of keeping data in ram depends on how ram you're using. trying find break point between cheaper keep data on disk or in ram.

however equation given incomplete. while equation identifies relevant factors, there important constant of proportionality missing. how many accesses can average hard drive sustain? how long ram last on average? enter costs keeping data on hard drives , ram, , without them comparing apples , oranges.

this indicative of impression of whole paper. says lot @ great length, important topic, analysis sloppy. slopping , leave critical things out, , don't enough people understand thinking , when analysis appropriate doing. instance if trying maintain low latency system, have keep of data in ram. period. if you're processing large data sets , don't want pay keep in ram, streaming data to/from disk. if you're keeping data in redundant format, instance raid, doing more seeks per read admit.


Comments

Post a Comment

Popular posts from this blog

Javascript line number mapping -

-
magically formatted comments change reported line number of javascript errors in browsers; this: //@line n "f" n line number , f file name. unfortunately, //@line appears ungoogleable. know there documentation on feature, , browsers support it? (i found references here , here .) from can find available in mozilla based browsers. it using js_setoptions command can turn these options on or off depending on values passed. https://developer.mozilla.org/en/js_setoptions
Read more

c# - Is it possible to remove an existing registration from Autofac container builder? -

-
something along lines: builder.registertype<mytype>().as<itype>(); builder.registertype<mytype2>().as<itype>(); builder.deregistertype<mytype>().as<itype>() var container = builder.build(); var types = container.resolve<ienumerable<itype>>(); assert.istrue(types.count == 1); assert.istrue(types[0].gettype == typeof(mytype2)); scenario: go through bunch of assemblies , go register types want make sure have 1 implementation of given type. need before create container. track on own nice if autofac me bit. this cannot done directly using containerbuilder , unless start on new one. mind you, having first built container should able construct new container filtering away unwanted types , reusing registrations first container. this: ... var container = builder.build(); builder = new containerbuilder(); var components = container.componentregistry.registrations .where(cr => cr.activator.limittype != typ...
Read more

php - Mysql PK and FK char(36) vs int(10) -

-
i have read mysql can handle indexes , searches better if tables keys integers rather char(36) uuid's. is worth converting int(10)? note: use 1 server , have have plans use multiple databases concurrently therefore removing 1 of big reasons using uuid. our tables innodb if makes difference. thanks in advance. usually rdbms can handle integer keys pk more efficient, other datatypes. reason how build index column, yes: long dont require string (or other typed) keys, should use integers. however: char(36) , int(10) far away being equal, because int(10) much smaller, char(36). dont know, if require many different keys, should keep in mind. update, complete last paragraph: int(10) 32-bit, char(36) 36 - ^byte^ (= 288-bit). not mean, int consumes less space, means, char(36) has 4 times more different keys.
Read more