Pipes sunexeia..

[Alexios Chouchoulas]

On Sun, 18 May 1997, Dimitrios P. Bouras wrote:

>   Kai ti shmainei "sync to file descriptor" otan to handle auto einai
>  enos FIFO?

  An katalabainw kala ta kernel sources, tipota apolytws. To sync tha
epistrepsei kanonikotata alla den tha graftei tipota ston disko kai to FIFO
den tha ephreastei katholou. O kwdikas gia pipes exei ola ta inode
operations kena.

  Epipleon meleth tou kernel leei ta ekshs: o buffer gia tis pipes einai 4k
(surprise surprise, 1 x86 page). Apo kei kai pera den mporw na katalabw 100%
ton kwdika (ti kathomai kai grafw... tetoia wra den mporw na ton katalabw
oute 40% pou einai to skor mou otan eimai akmaios). :-)


> >   Ta pipes (FIFO) etsi ki alliws den exoun dedomena pou grafontai ston
> > disko.
> 
>   Ontws, alla par'oti den exw diabasei ton kwdika tou kernel pou afora
>  sto sygkekrimeno 0ema, den mporw na fantastw to fdatasync() h fsync()
>  se fd gia pipe na kanei allo apo flush to pipe. Einai poly pi0ano na
>  kanw la0os, pragma pou 0a h0ela na to ma0w gia na dir0w0ei kai o
>  kwdikas tou xisp prin bgei h kainourgia ekdosh :)

  Ok, symfwna me ton kernoula (sic) mou, to fsync() einai Filesystem
operation, enw ta pipes einai memory structure (akoma kai ta named pipes). H
fsync fwnazei ena arxeio kai tou leei na grapsei ola ta inodes tou ston
disko. Dyo parathrhseis:

  (a) Den doulevei se ola ta filesystems (e, logiko einai -- merika, opws ta
ProcFS, NFS, SMBFS den exoun disko gia na grapsoun).

  (b) Oi pipes den exoun inodes, ara h entolh gia fsync() mpainei apo to ena
afti kai bgainei apo to allo.

  Kai to nohma ths istorias: thn pipa ki an thn fsync()iazeis, ta CPU cycles
sou xalas. :-)

  Twra pou to eipa afto, prepei na prosthesw oti mporei pia na kserw ti DEN
kanoun oi pipes, alla akoma DEN kserw pws akribws symperiferontai. Afto pou
kserw einai oti sto Solaris oi pipes einai non-standard kludgia kai einai
mia frikth empeiria na tis xeirizesai (eidika mesw NFS).


Alexios Chouchoulas          '88    ,88'   ,88'     alexios at vennea.demon.co.uk
The Unpronouncable One      ,o88oooo88ooooo88oo,              axc at dcs.ed.ac.uk

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