Messages in this thread | ![/](/images/icornerl.gif) | | Date | Sat, 30 Jun 2001 15:37:41 +0000 | From | Pavel Machek <> | Subject | Re: VM Requirement Document - v0.0 |
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Hi!
> Here's my first pass at a VM requirements document, > for the embedded, desktop, and server cases. At the end is > a summary of general rules that should take care of all of > these cases. > > Bandwidth Descriptions: > > immediate: RAM, on-chip cache, etc. > fast: Flash reads, ROMs, etc.
Flash reads aresometimes pretty slow. (Flash over IDE over PCMCIA...2MB/sec bandwidth. Slower than most harddrives.
> medium: Hard drives, CD-ROMs, 100Mb ethernet, etc.
CDroms are way slower than harddrives (mostly to seek times).
> slow: Flash writes, floppy disks, CD-WR burners > packeted: Reads/write should be in as large a packet as possible > > Embedded Case > ------------- > > Overview > -------- > In the embedded case, the primary VM motiviation is to > use as _little_ caching of the filesystem for reads as > possible because (a) reads are very fast and (b) we don't > have any swap. However, we want to cache _writes_ as hard > as possible, because Flash is slow, and prone to wear. > > Machine Description > ------------------ > RAM: 4-64Mb (reads: immediate, writes: immediate)
MB not Mb. 4Mb = 0.5MB.
> Flash: 4-128Mb (reads: fast, writes: slow, packeted) > CDROM: 640-800Mb (reads: medium) > Swap: 0Mb > > Motiviations > ------------ > * Don't write to the (slow,packeted) devices until > you need to free up memory for processes. > * Never cache reads from immediate/fast devices.
Flash connected over PCMCIA over IDE is *very* slow. You must cache it.
-- Philips Velo 1: 1"x4"x8", 300gram, 60, 12MB, 40bogomips, linux, mutt, details at http://atrey.karlin.mff.cuni.cz/~pavel/velo/index.html.
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