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SubjectRe: [Patch] shm bug introduced with pagecache in 2.3.11


On Fri, 12 Nov 1999, Manfred Spraul wrote:
>
> > 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.

Nope. The swap-in read-ahead still works - the _only_ thing the semaphore
does is serialize different processes accessing the same area, and that's
as likely to improve performace as to degrade it (potentially less
seeking).

> 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.

I will bet you 5 bucks we'll kick ass.

Linus


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