Messages in this thread | | | Date | Wed, 18 Jul 2018 18:31:15 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO |
| |
On Wed, Jul 18, 2018 at 09:56:33AM -0400, Johannes Weiner wrote: > On Wed, Jul 18, 2018 at 02:46:27PM +0200, Peter Zijlstra wrote:
> > I'm confused by this whole MEMSTALL thing... I thought the idea was to > > account the time we were _blocked_ because of memstall, but you seem to > > count the time we're _running_ with PF_MEMSTALL. > > Under heavy memory pressure, a lot of active CPU time is spent > scanning and rotating through the LRU lists, which we do want to > capture in the pressure metric. What we really want to know is the > time in which CPU potential goes to waste due to a lack of > resources. That's the CPU going idle due to a memstall, but it's also > a CPU doing *work* which only occurs due to a lack of memory. We want > to know about both to judge how productive system and workload are.
Then maybe memstall (esp. the 'stall' part of it) is a bit of a misnomer.
> > And esp. the wait_on_page_bit_common caller seems performance sensitive, > > and the above function is quite expensive. > > Right, but we don't call it on every invocation, only when waiting for > the IO to read back a page that was recently deactivated and evicted: > > if (bit_nr == PG_locked && > !PageUptodate(page) && PageWorkingset(page)) { > if (!PageSwapBacked(page)) > delayacct_thrashing_start(); > psi_memstall_enter(&pflags); > thrashing = true; > } > > That means the page cache workingset/file active list is thrashing, in > which case the IO itself is our biggest concern, not necessarily a few > additional cycles before going to sleep to wait on its completion.
Ah, right. PageWorkingset() is only true if we (recently) evicted that page before, right?
| |