lkml.org 
[lkml]   [1999]   [Nov]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [Patch] shm bug introduced with pagecache in 2.3.11
Linus Torvalds wrote:
>
> On Fri, 12 Nov 1999, Manfred wrote:
> >
> > I don't like the semaphore, because (AFAICS, I'm only looking at the diff)
> > you single-thread the swapin code (per-segment, but still single thread)
>
> I think the semaphore is a good idea, if only because it makes things much
> more obviously correct - exactly because of the clear serialization.

I agree that the current code is a total mess (I have converted it to
the ipc/util.h helper functions, and I found further SMP and UP races)
_if_ I find a simple serialization, then I'll kill the semaphore.

> And I don't think the serialization is a performance problem, because
> by the time you start paging we're not talking about high performance
> shared memory anyway, and because it's per-segment it is notgoing to make
> "system" performance any worse.
>
What about a 100-gigabyte shm segment (on a 64-bit platform) with a fast
scsi disk system? The semaphore will prevent any tagged commands, and it
will downgrade (performance wise) the scsi system to a slow ide disk.

Btw, I'm sure that for multi-threaded applications, the mmap performance
of Linux will be poor because everything is single-threaded. I'll
write a benchmark and compare it with WinNT/Win95.

>
> In fact, my reaction to the semaphore is "do we actually need the
> spinlock any more"?
>
shm_swap() must not acquire a semaphore, or we could lock-up during
low-memory.

--
Manfred


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:55    [W:0.076 / U:0.384 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site