On Wednesday 04 January 2006 12:13, Eric Dumazet wrote:
> Andi Kleen a écrit :
> > Eric Dumazet <[email protected]> writes:
> >> 1) Reduces the size of (struct fdtable) to exactly 64 bytes on 32bits
> >> platforms, lowering kmalloc() allocated space by 50%.
> >
> > It should be probably a kmem_cache_alloc() instead of a kmalloc
> > in the first place anyways. This would reduce fragmentation.
>
> Well in theory yes, if you really expect thousand of tasks running...
> But for most machines, number of concurrent tasks is < 200, and using a
> special cache for this is not a win.
It is because it avoids fragmentation because objects with similar livetimes
are clustered together. In general caches are a win
if the data is nearly a page or more.
>
> >
> >> + * read mostly part
> >> + */
> >> atomic_t count;
> >> struct fdtable *fdt;
> >> struct fdtable fdtab;
> >> - fd_set close_on_exec_init;
> >> - fd_set open_fds_init;
> >> + /*
> >> + * written part on a separate cache line in SMP
> >> + */
> >> + spinlock_t file_lock ____cacheline_aligned_in_smp;
> >> + int next_fd;
> >> + embedded_fd_set close_on_exec_init;
> >> + embedded_fd_set open_fds_init;
> >
> > You didn't describe that change, but unless it's clear the separate cache lines
> > are a win I would not do it and save memory again. Was this split based on
> > actual measurements or more theoretical considerations?
>
> As it is a refinement on a previous patch (that was integrated in 2.6.15) that
> put spin_lock after the array[] (so cleary using a separate cache line), I
> omited to describe it.
Ok, perhaps you should describe that too then
-Andi
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