Re: [PATCHSET v3 0/5] Support for RWF_UNCACHED

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On 12/11/19 10:56 AM, Jens Axboe wrote:
>> But I think most of the regular IO call chains come through
>> "mark_page_accessed()". So _that_ is the part you want to avoid (and
>> maybe the workingset code). And that should be fairly straightforward,
>> I think.
> 
> Sure, I can give that a go and see how that behaves.

Before doing that, I ran a streamed read test instead of just random
reads, and the behavior is roughly the same. kswapd consumes a bit less
CPU, but it's still very active once the page cache has been filled. For
specifics on the setup, I deliberately boot the box with 32G of RAM, and
the dataset is 320G. My initial tests were with 1 320G file, but
Johannes complained about that so I went to 32 10G files instead. That's
what I'm currently using.

For the random test case, top of profile for kswapd is:

+   33.49%  kswapd0  [kernel.vmlinux]  [k] xas_create                          ◆
+    7.93%  kswapd0  [kernel.vmlinux]  [k] __isolate_lru_page                  ▒
+    7.18%  kswapd0  [kernel.vmlinux]  [k] unlock_page                         ▒
+    5.90%  kswapd0  [kernel.vmlinux]  [k] free_pcppages_bulk                  ▒
+    5.64%  kswapd0  [kernel.vmlinux]  [k] _raw_spin_lock_irqsave              ▒
+    5.57%  kswapd0  [kernel.vmlinux]  [k] shrink_page_list                    ▒
+    3.48%  kswapd0  [kernel.vmlinux]  [k] __remove_mapping                    ▒
+    3.35%  kswapd0  [kernel.vmlinux]  [k] isolate_lru_pages                   ▒
+    3.14%  kswapd0  [kernel.vmlinux]  [k] __delete_from_page_cache            ▒

Next I ran with NOT calling mark_page_accessed() to see if that makes a
difference. See patch below, I just applied this on top of this patchset
and added a new RWF_NOACCESS flag for it for ease of teting. I verified
that we are indeed skipping the mark_page_accessed() call in
generic_file_buffered_read().

I can't tell a difference in the results, there's no discernable
difference between NOT calling mark_page_accessed() or calling it.
Behavior seems about the same, in terms of pre and post page cache full,
and kswapd still churns a lot once the page cache is filled up.

-- 
Jens Axboe




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