Message-ID: <aYu4xK8HhjO7mLP8@cmpxchg.org>
Date: Tue, 10 Feb 2026 18:01:24 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Chris Li <chrisl@...nel.org>
Cc: Nhat Pham <nphamcs@...il.com>, akpm@...ux-foundation.org,
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Subject: Re: [PATCH v3 00/20] Virtual Swap Space

Hi Chris,

On Tue, Feb 10, 2026 at 01:24:03PM -0800, Chris Li wrote:
> Hi Johannes,
> On Mon, Feb 9, 2026 at 6:36 PM Johannes Weiner <hannes@...xchg.org> wrote:
> > Here is the more detailed breakdown:
> 
> It seems you did not finish your sentence before sending your reply.

I did. I trimmed the quote of Nhat's cover letter to the parts
addressing your questions. If you use gmail, click the three dots:

> > > > The size of the virtual swap descriptor is 24 bytes. Note that this is
> > > > not all "new" overhead, as the swap descriptor will replace:
> > > > * the swap_cgroup arrays (one per swap type) in the old design, which
> > > >   is a massive source of static memory overhead. With the new design,
> > > >   it is only allocated for used clusters.
> > > > * the swap tables, which holds the swap cache and workingset shadows.
> > > > * the zeromap bitmap, which is a bitmap of physical swap slots to
> > > >   indicate whether the swapped out page is zero-filled or not.
> > > > * huge chunk of the swap_map. The swap_map is now replaced by 2 bitmaps,
> > > >   one for allocated slots, and one for bad slots, representing 3 possible
> > > >   states of a slot on the swapfile: allocated, free, and bad.
> > > > * the zswap tree.
> > > >
> > > > So, in terms of additional memory overhead:
> > > > * For zswap entries, the added memory overhead is rather minimal. The
> > > >   new indirection pointer neatly replaces the existing zswap tree.
> > > >   We really only incur less than one word of overhead for swap count
> > > >   blow up (since we no longer use swap continuation) and the swap type.
> > > > * For physical swap entries, the new design will impose fewer than 3 words
> > > >   memory overhead. However, as noted above this overhead is only for
> > > >   actively used swap entries, whereas in the current design the overhead is
> > > >   static (including the swap cgroup array for example).
> > > >
> > > >   The primary victim of this overhead will be zram users. However, as
> > > >   zswap now no longer takes up disk space, zram users can consider
> > > >   switching to zswap (which, as a bonus, has a lot of useful features
> > > >   out of the box, such as cgroup tracking, dynamic zswap pool sizing,
> > > >   LRU-ordering writeback, etc.).
> > > >
> > > > For a more concrete example, suppose we have a 32 GB swapfile (i.e.
> > > > 8,388,608 swap entries), and we use zswap.
> > > >
> > > > 0% usage, or 0 entries: 0.00 MB
> > > > * Old design total overhead: 25.00 MB
> > > > * Vswap total overhead: 0.00 MB
> > > >
> > > > 25% usage, or 2,097,152 entries:
> > > > * Old design total overhead: 57.00 MB
> > > > * Vswap total overhead: 48.25 MB
> > > >
> > > > 50% usage, or 4,194,304 entries:
> > > > * Old design total overhead: 89.00 MB
> > > > * Vswap total overhead: 96.50 MB
> > > >
> > > > 75% usage, or 6,291,456 entries:
> > > > * Old design total overhead: 121.00 MB
> > > > * Vswap total overhead: 144.75 MB
> > > >
> > > > 100% usage, or 8,388,608 entries:
> > > > * Old design total overhead: 153.00 MB
> > > > * Vswap total overhead: 193.00 MB
> > > >
> > > > So even in the worst case scenario for virtual swap, i.e when we
> > > > somehow have an oracle to correctly size the swapfile for zswap
> > > > pool to 32 GB, the added overhead is only 40 MB, which is a mere
> > > > 0.12% of the total swapfile :)
> > > >
> > > > In practice, the overhead will be closer to the 50-75% usage case, as
> > > > systems tend to leave swap headroom for pathological events or sudden
> > > > spikes in memory requirements. The added overhead in these cases are
> > > > practically neglible. And in deployments where swapfiles for zswap
> > > > are previously sparsely used, switching over to virtual swap will
> > > > actually reduce memory overhead.
> > > >
> > > > Doing the same math for the disk swap, which is the worst case for
> > > > virtual swap in terms of swap backends:
> > > >
> > > > 0% usage, or 0 entries: 0.00 MB
> > > > * Old design total overhead: 25.00 MB
> > > > * Vswap total overhead: 2.00 MB
> > > >
> > > > 25% usage, or 2,097,152 entries:
> > > > * Old design total overhead: 41.00 MB
> > > > * Vswap total overhead: 66.25 MB
> > > >
> > > > 50% usage, or 4,194,304 entries:
> > > > * Old design total overhead: 57.00 MB
> > > > * Vswap total overhead: 130.50 MB
> > > >
> > > > 75% usage, or 6,291,456 entries:
> > > > * Old design total overhead: 73.00 MB
> > > > * Vswap total overhead: 194.75 MB
> > > >
> > > > 100% usage, or 8,388,608 entries:
> > > > * Old design total overhead: 89.00 MB
> > > > * Vswap total overhead: 259.00 MB
> > > >
> > > > The added overhead is 170MB, which is 0.5% of the total swapfile size,
> > > > again in the worst case when we have a sizing oracle.

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