Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap

From: "Huang, Ying" <ying.huang@intel.com>
To: Chris Li <chrisl@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>,
	 lsf-pc@lists.linux-foundation.org,
	Johannes Weiner <hannes@cmpxchg.org>,
	 Linux-MM <linux-mm@kvack.org>, Michal Hocko <mhocko@kernel.org>,
	 Shakeel Butt <shakeelb@google.com>,
	David Rientjes <rientjes@google.com>,
	 Hugh Dickins <hughd@google.com>,
	Seth Jennings <sjenning@redhat.com>,
	 Dan Streetman <ddstreet@ieee.org>,
	Vitaly Wool <vitaly.wool@konsulko.com>,
	 Yang Shi <shy828301@gmail.com>, Peter Xu <peterx@redhat.com>,
	 Minchan Kim <minchan@kernel.org>,
	 Andrew Morton <akpm@linux-foundation.org>,
	 Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>,
	 Michal Hocko <mhocko@suse.com>,  Wei Xu <weixugc@google.com>
Subject: Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap
Date: Mon, 13 Mar 2023 09:10:18 +0800	[thread overview]
Message-ID: <87ttyp78xx.fsf@yhuang6-desk2.ccr.corp.intel.com> (raw)
In-Reply-To: <ZAu5z7j/oxnnR4/J@google.com> (Chris Li's message of "Fri, 10 Mar 2023 15:14:23 -0800")

Chris Li <chrisl@kernel.org> writes:

> On Fri, Mar 10, 2023 at 11:06:37AM +0800, Huang, Ying wrote:
>> > Unfortunately it's a little bit more. 24 is the extra overhead.
>> >
>> > Today we have an xarray entry for each swapped out page, that either
>> > has the swapcache pointer or the shadow entry.
>> >
>> > With this implementation, we have an xarray entry for each swapped out
>> > page, that has a pointer to the swap_desc.
>> >
>> > Ignoring the overhead of the xarray itself, we have (8 + 24) / (8 + 1) = 3.5556.
>> 
>> OK.  I see.  We can only hold 8 bytes for each xarray entry.  To save
>> memory usage, we can allocate multiple swap_desc (e.g., 16) for each
>> xarray entry.  Then the memory usage of xarray becomes 1/N.
>
> The xarray look up key is the swap offset from the swap entry. If you
> put more than one swap_desc under the one xarray entry. It will mean
> all those different swap_descs will share a swap offset.

For example, if we allocate 16 swap_desc for each xarray entry.  Then,
we can use (swap_desc_index >> 4) as key to lookup xarray, and
(swap_desc_index & 0x15) to index inside 16 swap_desc for the xarray
entry.

Best Regards,
Huang, Ying