Re: More Slowdown

[Hans Reiser <reiser@namesys.com>]

PFC wrote:

>
>     Just a few words about this "slowdown" thing.
>
>     I use linux-2.6.11-cko1-swsusp2 with reiser4 included. I won't
> upgrade to  a new version until Hans says the current one is at least
> as stable as it  was before starting the merge...
>
>     I get the "slowdown" once in a while, usually for 2-5 seconds,
> it's not  that annoying.
>
>     However :
>
>     If I understand well, when memory is needed (memory pressure),
> reiser4  triggers a flush and writes dirty pages to the disk in large
> quantities.
>     This has many advantages like not writing at all the temporary
> files  which already have been deleted, deferred allocation, etc, it's
> really  cool.
>
>     However when "memory pressure" (ie. need to free some RAM) occurs,
> it is  usually that I am doing something interactively, like opening a
> document  or starting some software. Or it might mean that the
> database server just  received a big query which needs some sorting
> space in RAM for instance.
>     Thus, "memory pressure" almost always means "I need memory NOW
> and  someone is waiting in front of their screen for it"...
>     And it is at this very moment that reiser4 has to flush (to free
> memory).
>     Thus the "flush storm", by nature, always happens when you don't
> want it  to happen. It almost never happens when you are away from the
> computer  taking a leak, for instance.
>
>     This is analogous to the problem caused to postgres by
> checkpointing...  the postgres guys implemented a background writer to
> solve this.
>     I wonder if reiser4 could do the same, ie. trickle down dirty
> pages to  free up memory before it is actually needed, to improve
> reactivity. There  is a balance to be found, of course, between
> flushing as late as possible  (to benefit from all the nifty reiser4
> features) and flushing earlier (to  avoid triggering the "flush storm").
>     Maybe this could be set via a few controls...
>         - tell reiser4 to try to keep X amount of RAM immediately
> available  without flush
>         - when the CPU is idle, and/or when the disk is idle, start
> flushing,  but stop doing it as soon as some CPU is needed, or a key
> is hit...
>         - just do frequent, small partial flushes which will keep good
> locality  of reference while being small
>         - do it at a lower priority so that the keyboard does not
> stop  responding !!!

I agree that smoothness needs working on.  After we merge.

>
>     Huh, well, that's it... what do you think ?
>
>
>
>> When fsyncing, Reiser4, to my understanding, isn't allowed to put
>> stuff in it's memory cache - it must put it to disk right away.  And
>
>
>     This is the whole point of fsync, yes.
>     Now, it's pretty stupid for evlolution to issue a fsync() on every
> pixel  move or whatever.
>     fsync() is for databases or things which must survive a hard
> system  crash...
>     evolution could as well have used fflush() and everything would
> have been  alright. Dumb.
>
>> For comparative purposes, I hear of consumer systems with 2 or 4 GB of
>> ram, and I know of brand new PC hard disks which have a throughput of
>> less than 5 MB/s (my hard disk maxees out at 20 MB/s on my PC.)  Do
>> you know the throughputs of your drives, and the size of the memory in
>> your machines?
>
>
>     Hum. A crap laptop harddrive will do 15 MBytes/s and a recent
> normal  desktop drive (7200rpm ATA or SATA) will do 50 MBytes/s or
> more...
>     If you get a lot less (like, 5 MB/s), you have a problem (DMA
> disabled,  etc)
>
>
>
>
PFC is right.