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mm: Disable watermark boosting by default

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What watermark boosting does is preemptively fire up kswapd to free
memory when there hasn't been an allocation failure. It does this by
increasing kswapd's high watermark goal and then firing up kswapd. The
reason why this causes freezes is because, with the increased high
watermark goal, kswapd will steal memory from processes that need it in
order to make forward progress. These processes will, in turn, try to
allocate memory again, which will cause kswapd to steal necessary pages
from those processes again, in a positive feedback loop known as page
thrashing. When page thrashing occurs, your system is essentially
livelocked until the necessary forward progress can be made to stop
processes from trying to continuously allocate memory and trigger
kswapd to steal it back.

This problem already occurs with kswapd *without* watermark boosting,
but it's usually only encountered on machines with a small amount of
memory and/or a slow CPU. Watermark boosting just makes the existing
problem worse enough to notice on higher spec'd machines.

Disable watermark boosting by default since it's a total dumpster fire.
I can't imagine why anyone would want to explicitly enable it, but the
option is there in case someone does.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Juhyung Park <qkrwngud825@gmail.com>
Signed-off-by: celtare21 <celtare21@gmail.com>
This commit is contained in:
Sultan Alsawaf 2020-03-28 13:06:28 -07:00 committed by Ksawlii
parent 09d9c4b0f2
commit 867fdfeb8a
1 changed files with 0 additions and 13 deletions
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13
mm/page_alloc.c
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@ -321,20 +321,7 @@ compound_page_dtor * const compound_page_dtors[NR_COMPOUND_DTORS] = {
*/
int min_free_kbytes = 32768;
int user_min_free_kbytes = -1;
#ifdef CONFIG_DISCONTIGMEM
/*
* DiscontigMem defines memory ranges as separate pg_data_t even if the ranges
* are not on separate NUMA nodes. Functionally this works but with
* watermark_boost_factor, it can reclaim prematurely as the ranges can be
* quite small. By default, do not boost watermarks on discontigmem as in
* many cases very high-order allocations like THP are likely to be
* unsupported and the premature reclaim offsets the advantage of long-term
* fragmentation avoidance.
*/
int watermark_boost_factor __read_mostly;
#else
int watermark_boost_factor __read_mostly = 15000;
#endif
int watermark_scale_factor = 10;
/*