Re: [PATCH v2] fs/select: add vmalloc fallback for select(2)

From: Vlastimil Babka
Date: Tue Sep 27 2016 - 04:13:33 EST

On 09/27/2016 03:38 AM, Eric Dumazet wrote:
On Mon, 2016-09-26 at 17:01 -0700, Andrew Morton wrote:

I don't share Eric's concerns about performance here. If the vmalloc()
is called, we're about to write to that quite large amount of memory
which we just allocated, and the vmalloc() overhead will be relatively

I did not care of the performance of this particular select() system
call really, but other cpus because of more TLB invalidations.

There are many other ways to cause those, AFAIK. The reclaim/compaction
for order-3 allocation has its own impact on system, including TLB flushes.
Or a flood of mmap(MAP_POPULATE) and madvise(MADV_DONTNEED) calls...
This vmalloc() would however require raising RLIMIT_NOFILE above the defaults.

At least CONFIG_DEBUG_PAGEALLOC=y builds should be impacted, but maybe
we do not care.

I doubt anyone runs that in production, especially if performance is of concern.