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X-OriginatorOrg: cern.ch X-MS-Exchange-CrossTenant-OriginalArrivalTime: 30 May 2025 09:03:29.3470 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: fe581962-16c0-4a5b-340e-08dd9f58d87a X-MS-Exchange-CrossTenant-Id: c80d3499-4a40-4a8c-986e-abce017d6b19 X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=c80d3499-4a40-4a8c-986e-abce017d6b19;Ip=[20.208.138.155];Helo=[mx3.crn.activeguard.cloud] X-MS-Exchange-CrossTenant-AuthSource: AM4PEPF00025F96.EURPRD83.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: GV0P278MB0066 Dear Cachefiles Maintainers, I've observed that when using cachefiles there is extreme performance degradation when the cache backing file (i.e. in /var/cache/fscache) is severely fragmented. For example, we have a 40GiB file in a NFS 4.2 mount (rsize of 1 MiB), that fully resides in the local (fs)cache and reading starts at ~16MB/s. Reading the backing file directly can be done at ~500MB/s - the hardware limit. The backing file has ~200k extends and resides on an etx4 formatted SSD. Using perf record, I found the culprit to be iomap_seek_hole (caused by cachefiles_prepare_read) and its descendants, which account for 98% of cpu time, which in turn is almost 100% of the wall time. So the root cause is that SEEK_HOLE is too slow when it has to traverse lots of extends, at least for ext4. I've verified the time it takes to SEEK_HOLE manually and found consistent results (66ms) which behave as expected: the further one starts into the file the faster the seek gets. This is also reflected in the read rate through cachefiles, which grows in a 1 over "remaining file size" manner until it achieves the hardware-limited speed near the end of the file. This should also affect all other filesystems, that search for holes in such a linear fashion - which I imagine is most, if not all, of them. These slowdowns will mostly affect fully cached files, exactly the case where one would expect/need the best performance. They are also exasperated by smaller rsize or when cache read/fill patterns induce a lot of fragmentation. Therefore I think it sensible to address this issue. My naive impression is that using fiemap should help mitigate the impact. One could still fall back on existing SEEK_DATA/SEEK_HOLE behavior, in case fiemap is unavailable. While I wouldn't mind (attempting) to contribute the necessary code, I'm not too sure that my non-existent kernel development skills would actually be helpful. In any case, I wanted to bring this to your attention such that you at least may ponder about it. Cheers Benjamin