QtMoko Virtual Memory
sylvain.pare at gmail.com
Wed Sep 1 15:10:27 CEST 2010
yeah long but thx!
2010/9/1 Gennady Kupava <gb at bsdmn.com>
> В Срд, 01/09/2010 в 12:38 +0200, David Garabana Barro пишет:
> > It's not true for me, and it's a common misunderstanding IMHO.
> This is not misunderstanding, this is just one of questions which can't
> have only one answer. For some tasks swap is good, for others - bad. I
> have strong (enough for me) arguments too. Yours are strong for you, so
> this is just matter of situation.
> > For testing, simply try to download tiles at zoom level 11, for the upper
> > zoom levels in tangoGPS. Better if you make it twice (moving on the map)
> > Without swap, it will catch all available memory, and FR will get really
> > slooooow, to the limit of appearing to hang, and even sometimes oomkill
> > start killing some random process.
> > With swap, linux can swap unused pages (other daemon pages, not tangogps
> > and tangogps will continue running, and FR will be responsible. You only
> > notice some 1-4 seconds slowdown from time to time, when pages are
> swapped out
> > swap is not only for "creating" more memory. If FR starts to massively
> > pages to swap, it will really SLOW things a lot, for sure (the same is
> true for
> > your PC)
> > But it will help *a lot* to have more memory available for running apps.
> > Think on swap as a place where put unused memory pages, and use real RAM
> > currently used apps or caching files from slow uSD
> > Swap will *ALLWAYS* help, but will help a lot more on a limited memory
> > as FR
> > Here  you can read more about what I'm saying.
> >  http://kerneltrap.org/node/3202
> Thanks for description, now my position:
> First of all, i do not use swap on any Linux system i am using with
> amount of memory >=512Mb. I even do not use swap on desktop where i have
> tmpfs mounted to /tmp.
> Having something published on kerneltrap is not meant to be only
> possible answer, i can provide an example. Few years ago Linus believed
> that moving everything to userspace is good idea and this is way to go.
> Now i see everything is included to kernel (devfs vs udev, evdev vs
> tslib, kms vs userspace mode switching). But i am still at the point
> that every possible thing should be in userspace. I think, both points
> of view very extensivly published to mailing lists and both have strong
> grounds :)
> First, about swap in general. IMO, swap were introduced in absolutely
> different context. In 80's memory situation were completely different.
> now, real situations.
> 1. main swap problem is in it's nature. it will push to disk
> less-frequently used pages from memory but use 'freed memory'. But for
> me it turned out that it is impossible to predict which page is useful
> which is not _in future_, and it turns out that freed memory on all
> modern systems used to keep relatively useless huge disk cache. it's
> question (for me) is _huge_ disk cache is better than _meduim-sized_
> disk cache in many situations. note that I found that using tmpfs is
> times faster than using such 'buffer' (i tested qtmoko build). having
> random process swapped-out also makes system very unpredictive. you may
> never know how long some application will start, this annoying for me, i
> like low latency.
> 1a. to apply (1) to FR. imagine you have phone app in background. it got
> swapped out as you did last call 8 hours ago. now, you recieve call and
> what? you should wait for your app to be paged back even to start
> hearing ring! considering fr sd io speed is 2.6M/s, and your app is for
> example 10M, this will take 4 seconds in best case. something have to be
> also discarded from memory too free up that 10M before loading your
> phone app.
> 2. situation (2) is described in  you pointed. i personally face it
> tons of times - some program (last were firefox and midming commander)
> just run out memory due to bug or other reasons and starts incrementally
> requesting swap. if you unlicky (and do not kill app in 30 seconds), you
> may get your terminal and most of X be unloaded to swap, and only thing
> you can do after that is hard reset. introducing limits will kill only
> really useful feature of swap - being able to load something larger than
> memory (3)
> 3. about your favorite gps application. i think it should create file
> and map it to memory instead of using extreme amounts of ram to store
> all data. In older day of swap not all systems had such ability. It's
> strange that sometimg became 'slow'. without swap it should never become
> slow, it should be just oomkilled. in fact, i do not understand this
> problem very well, as i think that just next malloc should return 0, or
> new throw bad_alloc.
> 4. swap on nand is special story. recently i saw really interesting
> article on this topic (by DocScrutinizer i think). as nand has block
> size of >4kb, writing (and reading) scatter pages (4k) to it may be very
> slow process.
> 5. if your distribution is on internal NAND, it may be several times
> faster to reload application from internal flash than to swap from sd.
> So, i can see that i only had problems with swap and never had real
> _need_ to use it.
> And, because of all this i am not using swap on any of my systems,
> including fr, so i have predictible and error-prone systems.
> PS. I have one other thing to rant laying near this question. I can see
> many statements that swap file is same to swap partition, but i can't
> believe it is true - fs appears between disk and memory management
> system. For magnetic hdd this means fragmentation, which can decrease
> performance below anything. Also, typical modern HDD has x2 (two times)
> speed difference between reading first and last sector, so it is
> important to place swap to proper location (depends on HDD).
> PPS. whooh, i knew that it will be long...
> Openmoko community mailing list
> community at lists.openmoko.org
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