Memory use

This page explains how the memory figures in the README and on flectar.com/mail are measured, and how to check them yourself.

Results

Measured on 5 November 2026 in one session, on the setup below: the same mailbox and window size, a fresh launch, the same HTML newsletter displayed and a sample 60 seconds after launch. Each range covers two runs. Every process an app starts is counted, and RSS is summed over them.

Memory (MiB)	Private	PSS	RSS	Processes	Renders email with
Flectar Mail	17.2 – 17.4	79.1 – 80.0	85.9 – 86.6	1	Blitz, CPU renderer
Geary 46.0	126.1 – 126.2	309.6 – 311.4	446.4 – 447.3	8	WebKitGTK 2.54
Evolution 3.56.2¹	191.0 – 192.2	395.0 – 395.9	697.5 – 698.4	12	WebKitGTK 2.54
Thunderbird 159.0	244.1 – 248.1	424.4 – 427.1	563.2 – 567.8	3	Gecko
Betterbird 153.6.0esr-bb12²	243.9 – 247.8	411.9 – 419.7	556.8 – 560.4	3	Gecko
Mailspring 1.26.2	311.4 – 314.9	551.1 – 551.7	1035.3 – 1036.4	10	Electron (Chromium)
Aerion 0.3.6	421.9 – 437.3	660.4 – 678.0	844.5 – 858.7	3	WebKitGTK 2.54

Includes the evolution-data-server services it starts: 39 MiB private and 72 MiB PSS. Evolution's own processes used 151.8 – 153.0 MiB private and 323.1 – 324.0 MiB PSS. GNOME desktops usually run these services already.
The first relaunch after its initial sync measured 277.8 MiB private and 456.2 MiB PSS; the range covers the two runs after it.

Engines and sandboxes. Geary and Evolution ran WebKit with its sandbox, so bwrap and xdg-dbus-proxy are counted. Aerion starts WebKit without it. Mailspring's count includes its mailsync process and Chromium's crash handler.
No GPU. The test machine has no graphics card, so every app rendered in software; Aerion's WebKit processes, for example, loaded Mesa's llvmpipe. Results on a desktop with a GPU may differ.
Connections. Flectar Mail, Geary and Evolution used IMAP over TLS. Thunderbird, Betterbird, Aerion and Mailspring used plain IMAP: Aerion and Mailspring rejected the test server's self-signed certificate.
Folders. All six folders were subscribed, so every client listed the same folders.
Fonts. The test machine has 8 font faces; a desktop usually has hundreds. With a 358-face font set, Flectar Mail measured 18.3 – 18.5 MiB private. The other apps weren't re-measured with it.

Geary and Evolution are the Debian 13 packages; Thunderbird, Betterbird, Mailspring (.deb) and Aerion (release tarball) are the vendors' Linux builds, each with a new profile and default settings.

Flectar Mail in other states

The same machine and mailbox, before and after the changes in #191, #192 and #194. Each figure is private memory from one run, or a range over two.

Flectar Mail (MiB)	Before	After
Welcome screen, no account	8.2	7.9
Inbox and newsletter, as in the results above	18.5 – 18.6	17.2 – 17.4
The same at display scale 2	26.1	20.8 – 20.9
First sync, inbox list, 60 s after the last write	20.2	14.7
First sync with the newsletter open, 60 s after the last write	25.3	20.5

After a sync, SQLite's page caches stay full and the allocator keeps the pages they freed. Flectar Mail now releases both once its mail core has been quiet for 10 seconds, and again after you leave Mail, Calendar, Files or Contacts for another product.
Display scale changes the message tiles: the email body is drawn at the screen's physical resolution, in tiles of about 512 device pixels with one tile of overscan above and below the viewport. At scale 2 the newsletter's tiles and other large buffers took 6.4 MiB, down from 10.6 MiB: tiles are now stored as RGB, without the alpha byte, and are half as tall in logical pixels there.
Threads. The D-Bus connection for the tray no longer starts a Tokio runtime with one idle thread per CPU core: the idle app now runs 11 threads instead of 22 on the 12-core test machine.
Low memory. When the system reports memory pressure, Flectar Mail drops what it can rebuild, such as message tiles outside the viewport, image caches and SQLite's page caches, and returns the freed memory (#201). Measured on that pull request's build with the newsletter open, a low-memory warning from the desktop portal took private memory from 17.5 to 16.2 MiB within five seconds.

On Windows 11, with 7 real accounts and about 69,000 unread messages, Task Manager's Details tab showed, before these changes:

Flectar Mail (MiB)	Private working set	Working set	Commit size
Window open, idle	37.8	64.5	53.7
Closed to the tray	4.2	8.3	49.4

Windows builds now also use the segment heap (#195) and decommit free heap memory when they trim (#191); neither has been measured on Windows yet.

The README's Performance table quotes these figures as measured, with how much more memory each app uses than Flectar Mail, comparing the means of each pair of runs. Its headline and summary round them to about 20 MB of private memory and about 85 MB with code and libraries (1 MiB = 1.049 MB): the runs above measured 18.0 – 18.2 MB and 82.9 – 83.9 MB, and a desktop with a full font set measures 19.2 – 19.4 MB of private memory.

What the numbers mean

Metric	Counts	Where you see it
Private memory	Memory that belongs to the app alone and isn't backed by a file: heap, caches, stacks. Adds up correctly across processes.	Windows Task Manager's Memory column (private working set); GNOME System Monitor's Memory column (RSS minus shared); `RssAnon` on Linux
PSS	Private memory plus the app's code and libraries mapped from disk, with shared pages split between the processes that use them	Plasma System Monitor's Memory column, `smem`, `/proc/<pid>/smaps_rollup`
RSS	Every resident page, shared libraries in full. Summed over several processes, it counts shared pages more than once.	`top`, `htop` (RES)

Most of Flectar Mail's RSS is its own executable: about 60 MiB of code and read-only data mapped from disk. Those pages are clean, so the system can drop them under memory pressure and read them back later.

Why Windows shows less than Linux

Different columns. Task Manager's Memory leaves out program code, while htop (RSS) and Plasma System Monitor (PSS) count the 60 MiB executable. Compared like for like, the large Windows mailbox uses more private memory (37.8 MiB) than the one-account Linux test (17.2 – 17.4 MiB).
More of the program file counts on Linux. Linux maps a file 64 KiB at a time around each page that's touched, so RSS includes code that may never run. That's one reason the Windows working set (64.5 MiB) is lower than the Linux RSS (86 MiB) despite the much larger mailbox.
The tray trim. When the window closes to the tray, or stays minimized for 10 seconds, Flectar Mail releases its rendering buffers and, on Windows, empties its working set. Windows moves those pages to its standby list, so the working set drops to a few MiB while commit size, the memory still allocated, barely moves. When the window reopens, the pages usually come straight back from RAM. Linux has no equivalent: closing to the tray frees the window's memory, but the program's code stays resident, and minimizing changes little.

How to measure

Keep everything but the app identical: machine, account, mailbox, window size and renderer. Use the same state every time: a fresh launch, the inbox open, a message displayed and no interaction for 60 seconds. Repeat each measurement at least twice.

Linux

scripts/generate-benchmark-mailbox.py ~/bench/Maildir   # test mailbox; serve it over IMAP
scripts/memory-snapshot.py "$(pidof flectar-mail)"
scripts/memory-snapshot.py --exe-dir /opt/thunderbird   # every process of an app

Windows

In Task Manager, open Details, right-click a column header and choose Select columns. Add Memory (private working set), Working set (memory) and Commit size. For an app with several processes, add up all of them; the Processes tab groups them for you.

Keep the window open while you read the numbers. After Flectar Mail has been minimized for 10 seconds, or closed to the tray, it releases its rendering buffers and trims its working set, so Task Manager then shows the background figure. Commit size counts all the private memory the app has committed, whether or not it is in RAM at that moment, so it doesn't drop after a trim.

macOS

Activity Monitor's Memory column shows the physical footprint, which also counts window surfaces held for the display, so it reads higher on Retina screens. footprint <pid> or vmmap --summary <pid> gives the breakdown.

Test setup

Flectar Mail: CI AppImages with the default CPU renderer. "After" is main at 51c6aed with #191, #192, #194 and #195 merged; "before" is 81d125b, the same code as main before them. The optional GPU renderer uses more memory and isn't covered here.
Other clients: the versions and builds listed under Results.
System: Debian 13, Linux 6.12, x86-64. Xvfb at 1920 × 1080 with Openbox, and every window at 1320 × 800 and scale 1. The scale-2 runs used a second Xvfb screen at 3200 × 2000.
Mailbox: Dovecot 2.4.1 on the same machine, serving the Maildir written by scripts/generate-benchmark-mailbox.py. It has 600 Inbox, 200 Archive and 60 Sent messages. About 45% are HTML newsletters, 20% short HTML notifications and 35% plain-text conversations, some with PDF attachments.
Windows: Windows 11 with 7 real accounts and about 69,000 unread messages, read from Task Manager's Details tab with the window open and again after closing it to the tray.

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